Why aren't zoom lenses faster than 2.8?

[elflord]

Generally the technical difficulty of achieving a particular geometry is INDEPENDENT of sensor size, meaning it's equally difficult to create a 17-55mm f/2.8 IS lens as it is to create a 24-105mm f/4.0 IS.

How about a 25mm f/0.7 lens for micro 4/3 ? Should that be comparably difficult to design and build as a 50mm f/1.4 full frame lens ?

To respond to your post though, there is NO benefit to a 1 stop faster aperture on APS-C sensor vs full frame because they (more than) cancel each other out. You don't stop action any quicker at all, whatsoever, because remember the ISO is skewed too, so all you're doing is just turning up the ISO sensitivity in a roundabout way. You have been misled into thinking there is a benefit.

The 400mm f/4 APS-C lens is "equivalent" to a 640mm f/6.4 lens. If you're distance limited, this really is preferable to a 400mm f/5.6 lens on full frame. You're right that it isn't "faster", but it is much longer than the 400mm f/5.6 full frame setup and almost as fast. Unfortunately, it's also quite a bit more expensive (not a whole lot cheaper to make the same lens for a smaller image circle)

[dolina]

Cost, weight and complexity.

[neuroanatomist]

Elflord is correct, but the practical consequence remains the same.  Yes, when you state the 'FF equivalent' of a P&S lens, that small aperture applies to DoF not exposure.  But when considering shutter speed, you also need to consider ISO noise, and the relatively greater total light gathered by the larger sensor means less noise.  For example, ISO 640 on my S100 (which has a big sensor, for a P&S) is equivalent to ISO 3200 on a FF sensor (at least), so for a given necessary shutter speed, the 'fast' lens with the small sensor has no advantage in terms of noise - you can get the same shutter speed + noise output on FF.

The 400mm f/4 APS-C lens is "equivalent" to a 640mm f/6.4 lens. If you're distance limited, this really is preferable to a 400mm f/5.6 lens on full frame. You're right that it isn't "faster", but it is much longer than the 400mm f/5.6 full frame setup and almost as fast. Unfortunately, it's also quite a bit more expensive (not a whole lot cheaper to make the same lens for a smaller image circle)

The only real advantage to APS-C when 'focal length limited' is more MP in the final image.  From an IQ standpoint, the FF image cropped to APS-C FoV will be equivalent (for similar sensor generations), so if the 7-8.5 MP of the cropped FF image is sufficient for the intended output, the 'reach advantage' of a crop sensor is an illusion.  Of course...if you need the MP (24x36" prints, for example), the advantage is real. 

[rs]

The "amplification" is not "turned up" for the smaller sensor. A 6x4mm sensor is no more noisy (or "amplified") than a 6x4mm region of a 35mm sensor. Images from the smaller sensor are (theoretically) about as noisy as a crop from a larger sensor.

The reduction in noise has to do with sampling density and averaging, not amplification.

The smaller the photosite, the less light it gathers - therefore less electrical signal is generated, so it will need a higher amplification before being fed into the A/D converter. If we are comparing the 12MP G15 sensor to a 4.6x bigger diagonal 12MP D3s sensor, not only will the FF sensor with 21 times as much area collect 21 times as much light, but each photosite (pixel) will gather 21 times as much light - which means 21 times less amplification is needed to get the same ISO's - so ISO 1600 on FF equals ISO 80 on the G15 in terms of noise.

However, if we're talking about a 12MP G15 vs a 253MP FF (that same pixel size scaled up to FF), the amplification should be identical. That scenario is where your statement of sampling density and averaging are correct.

Usually its a combination of the two as the number of MP are almost never identical in these comparisons, as are the size of each individual pixel.

ps - I was talking about comparing the entire imaging system - FF sensor, FF lens uncropped to a compact camera

[Hillsilly]

To respond to your post though, there is NO benefit to a 1 stop faster aperture on APS-C sensor vs full frame because they (more than) cancel each other out. You don't stop action any quicker at all, whatsoever, because remember the ISO is skewed too, so all you're doing is just turning up the ISO sensitivity in a roundabout way.

Ok, Thanks.  So, assuming similar generation sensors, an APS-C based camera at ISO 100 at 1/100s would have similar (maybe even worse) image quality to a FF camera at ISO 200 at 1/100s.  Therefore no real speed benefit.  You'd just have more MPs, which may o may not be important.  Makes sense.

Although, we might still have to disagree on the conversion of apertures to 35mm equivalents. While converting the focal length to a 35mm equivalent makes sense (with the preponderance of different sensor sizes, it's good to have a commonly accepted way of understanding focal length), converting the aperture causes confusion.  At the same focus distance, a 30.5mm f/2.8 on a G15 has essentially the same background blur and requires the same shutter speed as a 30.5mm f/2.8 lens on a 7D and a 30.5mm f/2.8 lens on a 1Dx. In other words, a 30.5mm f/2.8 lens doesn't change any of its attributes when the sensor changes in size.   Therefore, there is no need to convert it to a 35mm equivalent.   The 30.5mm f2/8 is already a 35mm equivalent for every exposure and depth of field measurement.

[neuroanatomist]

Although, we might still have to disagree on the conversion of apertures to 35mm equivalents. While converting the focal length to a 35mm equivalent makes sense (with the preponderance of different sensor sizes, it's good to have a commonly accepted way of understanding focal length), converting the aperture causes confusion.  At the same focus distance, a 30.5mm f/2.8 on a G15 has essentially the same background blur and requires the same shutter speed as a 30.5mm f/2.8 lens on a 7D and a 30.5mm f/2.8 lens on a 1Dx. In other words, a 30.5mm f/2.8 lens doesn't change any of its attributes when the sensor changes in size.   Therefore, there is no need to convert it to a 35mm equivalent.   The 30.5mm f2/8 is already a 35mm equivalent for every exposure and depth of field measurement.

But...if the 1D X is taking a head shot, the 7D is cutting off the chin and hair, and the G15 is giving you just the eyes and nose - not exactly a flattering portrait.  A more relevant comparison is similarly-framed shots, and then the 'crop factor' effect on DoF applies (because with a smaller sensor and the same lens, you're further from the subject for the same framing).

[rs]

Although, we might still have to disagree on the conversion of apertures to 35mm equivalents. While converting the focal length to a 35mm equivalent makes sense (with the preponderance of different sensor sizes, it's good to have a commonly accepted way of understanding focal length), converting the aperture causes confusion.  At the same focus distance, a 30.5mm f/2.8 on a G15 has essentially the same background blur and requires the same shutter speed as a 30.5mm f/2.8 lens on a 7D and a 30.5mm f/2.8 lens on a 1Dx. In other words, a 30.5mm f/2.8 lens doesn't change any of its attributes when the sensor changes in size.   Therefore, there is no need to convert it to a 35mm equivalent.   The 30.5mm f2/8 is already a 35mm equivalent for every exposure and depth of field measurement.

If they're all cropped to the same angle of view, and printed at the same size. If not, we're talking about very different systems which cannot be compared in such simplistic terms.

If you use the effective focal length in 35mm terms to represent angle of view, lets get one thing straight - the aperture has to be adjusted too, otherwise you're no better than Panasonic's marketing department were with their FZ200:

[Radiating]

I resized and re-uploaded the photo as the link doesn't work for embeding unless it's in your cache.



Hillarious though!

Personally I would like to see the following lenses from Canon:

20-35mm f/2.0
35-70mm f/2.0
70-160mm f/2.0

These lenes would all be possible as they are aproximately 2x zooms and would have 82mm filter sizes

[RGF]

Consider the size of 2 or 1.4 zoom lens.  Take the focal length and divide by the aperature.  That is a rough guide as to the size of the front lens.  The exact definite of the F stop is the focal length divided by the size of the aperture.  So for a 70-200 F2 lens, the aperture would need to be 100 mm - take this to be the size of the front element.  82 mm filters are expensive enough, 100mm filters would be mucho $$

Well my $0.02 and the limit of knowledge (probably directionally correct but only approximately correct) of optics.

[rs]

I resized and re-uploaded the photo

Thanks

[qwerty]

Has anyone tried actually searching for patents / designs for zoom lenses that are faster than f/2.8 35mm equivalent so that we could get a realistic idea of the size and/or cost?  Surely someone out there has made such a design.

I spend about 30 seconds checking patents for fast zooms, but didn't want to waste the time searching for the crop factor when someone else out there can probably do a better and faster job than I could.

For what its worth, I would probably only pay $2000-2500 for a 35-85mm f/2 that weighed a little over 2 lbs and was comparable to their 24-xxx zooms in image quality (the 24-70 ii is about $2050 currently on amazon).  My wild and speculative guess is that if Canon were to actually introduce such a lens it would be more than double that price, so its kind of a moot point for me.  I would still be interested in seeing design info though.

[rs]

Has anyone tried actually searching for patents / designs for zoom lenses that are faster than f/2.8 35mm equivalent so that we could get a realistic idea of the size and/or cost?  Surely someone out there has made such a design.

Here you go: http://www.adorama.com/HSHC50110.html

Hasselblad MF lens 50-110/3.5-4.5, designed for a 44x33mm sensor (0.76x crop). In 35mm speak, its equivalent of a 38-84/2.66-3.42

$5,595, 1.6kgs, 152x103mm, 95mm filters.

[Hillsilly]

Pentax 67 55-100mm F/4.5.  Works out to be a 35mm equivalent f/2.2 zoom.  There would be a few more floating around in medium format land.

[preppyak]

For what its worth, I would probably only pay $2000-2500 for a 35-85mm f/2 that weighed a little over 2 lbs and was comparable to their 24-xxx zooms in image quality (the 24-70 ii is about $2050 currently on amazon).  My wild and speculative guess is that if Canon were to actually introduce such a lens it would be more than double that price, so its kind of a moot point for me.  I would still be interested in seeing design info though.

Yeah, I don't think its a technical limitation, it's more an R&D time limitation that keeps them from making f/2 zooms. As others have mentioned, you'd need a lot of glass and large filters, which by weight and filter size eliminates a lot of the market. Then you have to recoup the cost of R&D, and the cost of all that glass, so your starting point price is already higher than $2500+ I'd bet. Then, because its expensive, fewer people are in the market. Forum kings will complain they can have a 35L, 50L, and 85L for the price, and they are all faster than f/2! So your market grows smaller, and cost goes up. Assume it's a $4000 zoom; who is really buying it over the 24-70 f/2.8 at half the price? Those that NEED f/2 also probably NEED f/1.4 as well.

It's also looking to solve a problem that basically doesn't exist, and that pros likely won't pay for. They'll carry a 35L and an 85L, or a 24-70 and a prime instead. Whereas, with the 200-400, getting a 200-560 zoom that is super high quality and doesn't necessitate lens changes in bad environments is something a pro will pay for. If twice as many paid for it, the lens might cost $8-9000, instead of $11,000+.

[Hillsilly]

Sigma have announced a new 18-35mm F/1.8 zoom lens.  It's for APS-C sensors, but it shows some manufacturers see a market for fast zooms.