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Messages - rs

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This is a compound error from the previous incorrect statement. Sensor size is not a factor in this.
My original argument which has been hotly debated was merely that as nice as this Sigma 18-35/1.8 is on a crop camera, it's not quite as nice* as a 24-70/2.8 II is on a full frame camera.  I was comparing two complete systems, sensor and all. I know that a 24-70/2.8 is no match for this Sigma when they're both mounted on a crop body.

*by nice, I mean the 24-70 on FF goes wider, longer, is capable of a vaguely narrower DoF and capturing vaguely more light.

My argument is simply the total quantity of light a system can capture is more than just aperture - it is a combination of aperture and sensor size. The entrance pupil size when both systems have an equivalent FoV is a simple way of quantifying that.

But if you disagree with me and think that an f1.8 lens on 1.6x crop captures more light than a lens with an equal field of view at f2.8 on full frame, then fair enough. I've tried explaining this concept in many different ways, and you still don't get it. So I give up.

I've decided to offer some help to Sigma with an FAQ set for potentially confused customers.


Think of the metabones speed booster. Imagine now that they created one which made a FF lens create an imaging circle to match the APS-C crop sensor found in a Canon camera - a 1.6x telecompressor. That way you could mount, say, a 24-70/2.8 on crop with the metabones adapter, and get an identical FoV that the lens achieves on FF.

The 24-70/2.8 would be turned into a 15-44/1.75, right?

Now, we all know f1.75 is faster than f2.8. No-one is disputing that. If you mount this lens on the crop sensor camera with the telecompressor, it will allow for more than a stop faster shutter speeds at equal ISO's. It is an f1.75 lens, and no-one can argue with that. But where does this metabones get this extra speed from? Its not magic - its just it compresses the larger image circle into a smaller one - that extra light from that larger FF image circle is now condensed down into a smaller, more intense imaging circle, and is then received by a smaller sensor. However, in total its only the same amount of light/photons coming in through the lens which hits the sensor. The FF sensor and the APS-C sensor with a telecompressor both receive an identical number of photons, but the APS-C sensor has brighter light presented to it - more light per area - in other words its just over a stop brighter.

ISO's are rated to make exposure calculations work. What one camera does to achieve ISO 1600 isn't the same as another camera at ISO 1600 - especially with different size formats. The larger sensor as a whole has more photons hitting it at a particular aperture, so it needs to amplify the resulting electrical signal less for any given ISO. And even some cameras with the same sensor have to do different amplification, such as the Sony NEX 7 and Sony SLT A77.

If you can't see that, haven't you ever wondered why FF sensors are typically just over a stop better than crop sensors when it comes to noise? This faster aperture of the Sigma simply allows the noisier sensor to work at lower ISO's to finally fight back. Use a 18-35/1.8 at 35mm f1.8 1/100th of a sec ISO 10000 on crop, or a 24-70/2.8 at 56mm f2.8 1/100th, ISO 25600 on FF and you'll find its the same framing, depth of field, exposure and noise. (well, it would be if the Sigma was slightly brighter at f1.75)

Regardless of how you understand this difference between full frame and crop, and either agree or disagree with me, this Sigma lens really ups the game for crop users. From the specs point of view (and samples images), it looks great.

EOS Bodies / Re: 21mp Sensor in the 7D Mark II? [CR1]
« on: April 19, 2013, 06:01:44 PM »

I hope for the best about 7D2: new dual DIGIC 6 and new CMOS (NEW Technolgy, NOT the same CMOS as on 1DX, 5D3 edition customized for 7D2).


No need to be "furious with Canon".  And where did you read a rumor that the 7D2 would get "dual digic 6"?  I must have missed that.  Seems very unlikely to me, unless the price is more in the $3k+ range.  Which would be absurd...a 1.6x crop sensor is a silly compromise in the first place (at least now in 2013).  To attempt to sell one at such a high price, would be a disaster.  1.6x crop, is so very 2002...it's time to move on to something else, something bigger.  I mean, if "reach" is all they care about, they might as well go smaller and make a micro 4/3 size, 3:2 sensor for the 7D2.  But they won't.

you have not a clue what you are talking about, as long the pixel density is higher in a APS than a 24x36 there are many people who has advantage of the smaller sensor, bird photographer, etc

You can compare what you want, as long as you it correctly. The way you do it is the way you can compare angles of view. This is fine as long as you don't make the mistake to consider angle of view = focal length and to put this value into a f/stop calculation.
Angle of view is a combination of focal length and sensor size. If we're talking about two different sized sensors, to get the same angle of view, we have to adjust the focal length to suit.

Focal length is what it is. You can mount an old Zeiss for Hasselblad lens on your Canon camera and the focal length will not change. It would give you the same framing of a 100mm Canon lens.
You're looking at it the wrong way here - while the 100mm Zeiss frames the same as the 100mm Canon on the Canon SLR, it frames very different from how it would on the Hasselbald. On the Canon, you've cropped out a fair old proportion of the image circle, resulting in much of the light being cropped out and a narrower FoV.

More flow, but not more pressure. Same with light: you get more light coming in in total, but the amount of light / surface of the sensor would be the same. Any extra light that comes in will not affect exposure.
That's my exact point - the intensity of the light at any area of the sensor isn't greater, its just there's a bigger area of it, so in total more light is captured by the system.

Same as above. A smaller sensor is actually a smaller mouth to feed. This is why you can have compact cameras with 1" sensor and f/1.8 lenses.
Again, it looks like you've nearly got it here - it does need that faster aperture to make it equivalent.


You are mixing two concepts that are unrelated.

If we talk about framing, then you're right. The smaller effective aperture is the reason why DoF is bigger on crop at a given aperture and angle of view (note that I didn't say focal length).

However, framing has nothing to do with light gathering. f-stops are a function of focal length, not angle of view. A 50mm is a 50mm on every camera. It's the angle of view that changes in relation to sensor size, not the focal length.

Another quote from Wikipedia:
A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil is larger than that of the 100 mm lens, but given the same light transmission efficiency, both will produce the same illuminance at the focal plane when imaging a scene of a given luminance.
But to compare a crop lens, crop sensor combo to a FF lens, FF sensor combo, there's no point in comparing two with different framing - otherwise you'd be arguing this 18-35 crop lens is a direct equivalent of a 16-35 FF lens on FF.

To exaggerate, is a 100mm f5.6 large format lens with its huge image circle the same as a 100mm f5.6 lens and its tiny image circle on a compact? Is it wrong to compare lenses which give the same framing? Surely from a photographers point of view, they're two very different lenses?

Wikipedia doesn't take into account imaging circle in that equation you're quoting. The aperture of a lens is a bit like working out the speed of water in a hose pipe. The imaging circle is a bit like the diameter of the pipe. Widen the pipe and keep the speed the same, you get more coming through.

Or think about it like this - imagine a photo taken with a FF lens and a FF sensor. Now you take that same photo and you crop out just the centre 40% - you've taken away 60% of the image - which is also 60% of the light that passed through that FF lens. You're left with only 40% of the light. That's what crop does. You need a faster lens on crop to make it capture the same amount of light in that smaller area.

Hey guys, read what I wrote. I said f1.8 on crop is brighter than f2.8 on full frame when both have the same shutter speed and ISO, then you start telling me that I'm wrong to say FF is brighter when they're both at f1.8 and the same shutter speed and ISO. I didn't say that, so what gives? I went to great lengths to explain that at the same aperture, shutter speed and ISO, they both expose the same due to the different light gathering of the format being compensated for by the amplifiers being set different. If you can look past that same exposure settings between formats and start to use the higher ISO's with lower noise levels this lower amplification of FF rewards you with, you'll find the true nature of the light gathering of FF lenses on FF sensors.

80mm 1.0x [FF]      @ f/2 aperture = 40mm diam pupil
50mm 1.6x [APSC] @ f/2 aperture = 25mm diam pupil

He is not.

Quote from: Wikipedia
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.

You can't compare different focal lenghts. A 50mm is a 50mm both on crop and FF. However, being the former smaller, you crop the edges to a degree which would be equivalent to the angle of view of a 80mm on FF. This is a quick way to grasp the concept, but it's not exact in a way that you can make calculations.
Thanks indigo9, nice explanation. Albi86 - a 1.6x crop camera crops the FoV, so to get the same framing, you have to use a different focal length. This 18-35 crop lens is a rival to a 24-70 on FF, not a 16-35. 80mm on FF does frame the same as 50mm on crop - and if they both have the same aperture, the 80mm lens has to have a bigger entrance pupil.

And using indigo9's simple (yet still misunderstood) explanation, to compare the 18-35/1.8 to the 24-70/2.8 on FF we'd need to set them both to a focal length to give an equal field of view. So for the sake of this example, lets use the long end of the Sigma's zoom - set the 18-35/1.8 to 35mm, which is equivalent of 56mm on the 24-70. We get the following:

56mm 1.0x [FF]      @ f/2.8 aperture = 20mm diam pupil
35mm 1.6x [APSC] @ f/1.8 aperture = 19.4mm diam pupil

So, vaguely less light gathering from the new Sigma lens, as well as a vaguely wider DoF.

Neuro - do you want to chime in at this point? People respect your opinion  ;)

And as I said before though, this is a very interesting lens, and its a great option for crop sensor users - while it lets in slightly less light and has a narrower zoom range than the 24-70/2.8, its a great alternative to moving to a more expensive body, and the internal zoom is a great feature for a normal lens.

Aperture is just a way to measure the diameter of the iris blade. It's connected to, but it doesn't measure, the real amount of light gathered. This is why T-stops were invented.
T-stops are a measured version of light as opposed to f stops which are theoretical - so they take into account the transmission of light, including effects such as reflections and tinting of glass. However, even T-stops don't take into account the size of the imaging circle or the size of the sensor. That's why a 24-70/2.8 II on FF is more than a worthy rival to this 18-35/1.8 on crop, yet if you mount the same 24-70/2.8 II on crop, it is not.

The reason why using an f1.8 lens wide open on crop gives a brighter image than f2.8 on FF (when both are at the same ISO and shutter speed) is the amplification of the crop cameras sensor is 2.56x greater, at the expense of noise at any given ISO rating.

What? That's wrong.

A crop f/1.8 lens and a full frame f/1.8 lens will provide exactly the same exposure when used at the same shutter speed and ISO. The full frame exposure WILL NOT be brighter.

You're right about an FF f/2.8 lens having more light gathering ability than a crop f/1.8 lens, but all that light it gathers is spread over a larger sensor, which makes the exposure more than a full stop darker than if you had used an f/1.8 lens.

So in terms of exposure, a f/1.8 lens is brighter than an f/2.8 lens, regardless of sensor size. Sensor size does affect depth of field, but that's a different story.

Exactly. Try it for yourself on a crop body and FF. f1.8 at 1/30 sec at 100 ISO will give you the same exposure on both cameras. FF will not be brighter.

Hey guys, read what I wrote. I said f1.8 on crop is brighter than f2.8 on full frame when both have the same shutter speed and ISO, then you start telling me that I'm wrong to say FF is brighter when they're both at f1.8 and the same shutter speed and ISO. I didn't say that, so what gives? I went to great lengths to explain that at the same aperture, shutter speed and ISO, they both expose the same due to the different light gathering of the format being compensated for by the amplifiers being set different. If you can look past that same exposure settings between formats and start to use the higher ISO's with lower noise levels this lower amplification of FF rewards you with, you'll find the true nature of the light gathering of FF lenses on FF sensors.

If you really think you can freely quote focal lengths in 35mm equivalent without quoting apertures in 35mm equivalents, then you're sounding very much like the Panasonic marketing department:

Surely you know that taking this small sensor, large aperture thing to extremes like this does not result in this Panasonic FZ200 having a lens equivalent of a 600mm f2.8 at full zoom, as Panasonic would like you to think? In terms of framing, yes. In terms of exposure due to ISO tweaking of the sensor, yes. In terms if DoF, no. In terms if light gathering, no! There's no way that a 'slow' Canon 600/4 II on a 1D X as a package gathers less light than that 'f2.8' lens/sensor combo of the Panasonic. Yes, both at f4, 1/1000th of a sec, they'll both have to use an identical ISO to expose the same, but look past using the same rated ISO and guess which combo would work best to get clean images at high shutter speeds in low light?

If you can understand that, then surely you can see the 1.6x crop gives just over a stop less light gathering than a full frame sensor - making f1.8 on crop equal in light capturing terms to an aperture 1.6x smaller on FF - f2.88.

I have them both, and there's really nothing in it at normal focusing distances. If only 100mm will do or I need to get very close, I'll take the lighter lens. They're both equally spectacular for portraits at 100mm, but I personally prefer to change focal length a bit to go allow me to take both tight head portraits and upper body group shots without having to change perspective too much - so invariably I find myself using the zoom.

This is great for crop shooters.  However, before everyone gets excited over the f/1.8 bit, you have to remember that f/1.8 on a crop sensor is nothing like f/1.8 on a FF sensor.  This lens will give the same angle of view, image noise for given exposure parameters (*1), depth of field at a given AOV and subject distance (*2), etc. etc. etc. as a 28-50mm f/2.8 full frame lens.

In other words, if the lenses and sensors are perfect, this lens on a crop sensor would give identical results to a 28-50mm f/2.8 on a FF sensor. 

You mean identical DOF. f1.8 will still give more light on APS-C than f/2.8 on full frame.
No. A FF sensor behind an f2.8 FF lens gathers 2.56x as much light as an f2.8 lens does on a 1.6x crop sensor due to the sensors 2.56x bigger surface area. If you only capture a fraction of all that FF f2.8 light by cropping it, well, the obvious happens from the light gathering point of view. The reason why using an f1.8 lens wide open on crop gives a brighter image than f2.8 on FF (when both are at the same ISO and shutter speed) is the amplification of the crop cameras sensor is 2.56x greater, at the expense of noise at any given ISO rating. In other words ISO 10,000 on crop is pretty much equal to ISO 25,600 on FF in terms of noise. So feel free to shoot smaller apertures on FF and use higher ISO's to get the same light gathering and noise.

The different amplification levels is a bit like how the Sony NEX 7 and the Sony SLT A77 both share an identical sensor, and give the same exposure with the same shutter speed, aperture and ISO, yet the SLT camera has a semi translucent mirror permanently in front of the sensor, acting like a neutral density filter you can't get rid of. Sony just cranked up the amplifier on the A77 a bit more to make it all seem good - at the expense of noise.

So f1.8 on a 1.6x crop is equivalent to f2.88 on FF in terms of both depth of field and light gathering.

I see this Sigma lens as being a crop alternative to the 24-70 II on FF in just the same way as the 17-55 IS is a crop alternative to the 24-105L on FF. Not quite as fast an effective aperture, and not as wide or as long effective focal lengths. Slightly worse on all fronts when compared to the FF equivalent, but none the less a nice string in the bow for crop sensor users. Lets hope it performs well optically, and I like the idea of an internal zoom on a lens covering the normal range, even if the zoom ring rotates the wrong way :)

Lenses / Re: Speedy Manual Focusing?
« on: April 18, 2013, 11:49:37 AM »
The EG-S is great and I leave it in my 5DMkII all the time. Viewfinder brightness is comparable to that of the 7D and I don't ever find the viewfinder to be too dark. IMHO these FF camera's should come with the EG-S as standard.

How does the EG-S screen work exactly?  I'm only familiar with the split prism manual focus screen.

Is the EG-S difficult to install?


The normal EG-A screen has micro lenses that are aimed at the center of the frame to produce a bright viewfinder (but also too much DOF), the precision matte is more like ground glass and much more diffuse. I can't quite explain how it works optically, but when you look through the screen it is much easier to see what is in focus and what isn't. The EG-S effectively shows a shallower DOF.

This site ( http://www.focusingscreen.com/work/5d2en.htm ) shows how to change focus screens, I would recommend a Canon EG-S over a third party one as a third party screen may interfere with the metering accuracy of your camera (notice that you need to change a setting in the camera to account for the different screen in use).

The brightness will really drop off with slower lenses. If you have an f5.6 or f8 lens (or stop your lens down to that and use the depth of field preview button), the EG-S screen will be a lot darker than a normal focus screen.

The rougher, or more matte the focus screen is (such as an EG-S focus screen) the more it can pick up on the off-angle light rays, such as you'd get with the converging light rays from a wide aperture lens. If a focus screen is smoother, such as the stock screen, the artificial 'ground surface' of the plastic screen doesn't have sufficient depth or angles on it to pick up on such off-angle light. The bulk of the light it receives is the stuff close to perpendicular to the focus screen - which explains both why these stock screens don't get as dark when stopped down, and why they can't render the out of focus parts of the image as blurred as the sensor can. Somewhere around f4 to f2.8 is where a stock focus screen fails to show how narrow the DoF is. If you use an f1.2 lens wide open with a standard focus screen, the viewfinder will be very different to the final image.

In response to the original question, focus screens such as the matte EG-S or stock EG-A give the manual focusing user the equivalent info (albiet scaled down) of what contrast detect AF has - no indication of exactly how much it's out by, or any real info about which way it's out. Plus without resorting to magnified live view, you can't accurately see if its 100% there, even with an EG-S screen. But you can hunt around, finding where it looks sharpest and opt for the middle of that acceptable range, hoping for the best if you're only using the non magnified viewfinder. However, a split image focus screen is much like a single point phase detect AF system - the amount and direction of focusing needed to get it spot on are represented by how far out of alignment and which way out of alignment the two parts of the split image are. With not much practice, accurate and fast manual focusing us possible with a split image focus screen.

Lenses / Re: Why aren't zoom lenses faster than 2.8?
« on: April 18, 2013, 08:13:08 AM »
There are now:

Equivalent of f2.7 FF on a Nikon 1.5x crop, or f2.9 FF on a Canon 1.6x crop. So on Nikon it is faster than an equivalent of f2.8 FF, but not by much. I'd still prefer a Canon 24-70 II on FF, even if it is 'only' f2.8

Lenses / Re: 2 filters jammed together
« on: April 12, 2013, 03:48:44 AM »
A rubber band stretched around a filter makes it much more grippy. Hopefully there's enough width for a rubber band on the half of the CPL which is tightened onto the UV filter.

Lenses / Re: Why can a Sigma 10-20mm fit on a "FF" Body?
« on: April 05, 2013, 04:42:01 AM »
While some manufacturers such as Sigma make lenses designed for Canon crop bodies, only Canon make EF-S mount lenses. This means these third party EF crop lenses can physically mount on FF bodies, even though the image circle isn't specifically designed to cover such a large area. Hopefully they have enough mirror clearance too.

Does the Sigma 10-20 image circle cover the entire frame when zoomed in to say 15/16mm or longer?

EOS Bodies / Re: 6,5 fps with 40D
« on: April 03, 2013, 04:32:57 PM »
My 40D is certainly the same speed with or without the grip and its extra battery. The only improvements the grip gives me are handling and battery life.

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