Just wanted to chime in on the conversion debate.
A 15-35mm f/1.8 lens on crop, will deliver identical optical geometry to a 29-56mm f/2.88
lens on full frame.
Depth of field for example, just like angle of view is a function of geometry.
So as the iris or aperture opens up more it allows more light in, but because that light is light that is coming at a higher incident angle it is focused over a greater area if it is behind or infront of the focal plane. So therefore you have a shallower depth of field. See how the red point is larger and therefore will be blurred on the open aperture example?
Here's another example:
People have been misled with crop to full frame conversions for years.
The "35mm equivalent" is what is really important and nothing else. Your images on 35mm equivalent will always look the same no matter what.
From a physics perspective the "35mm equivalent" is capturing identical information. What really matters is the geometry of the light hitting the sensor, and with 35mm equivalent the geometry will always be the same for a given equivalence. Not only that but your flash settings etc will be identical:
Going back to the 35mm equivalent discussion, consider this:
On APS-C (like the 7D) compared to full frame (like the 5D Mark III)
The sensor is 1.6 x 1.6 times smaller.
35mm equivalent aperture - Multiply F-Number by (1.6 ) . (an f stop is a base 2 log, so even though we have 1.6x1.6 times as much light we take the square root, which is 1.6 to multiply the F number by. (example 2.8 x 1.6 = 4.48, 4.0 x 1.6 = 6.4, 1.8 x 1.6 = 2.88))
35mm equivalent focal length - Multiply by 1.6
35mm equivalent ISO or light sensitivity - Multiply by (1.6 x 1.6) (bet you haven't heard of that, but if you do the math the an APS-C sensor amplifies the signal 1.6x1.6 times more at a given PIXEL than the a full frame camera, so even if both say ISO 800, ISO 800 on the an APS-C it is multiplying the light from each individual pixel the same as ISO 2000 full frame, assuming they had identical resolution.
This is an important point of contention. So while for a given area of a light sensetive material, ISO provides a set level of amplification, cameras do not have a consistent area that they absorb incoming light on. Some have finger nail sized sesnors, and some postage stamp sized sensors. What's more important to consider is each individual pixel and it's level of amplification. Images are formed from pixels, so to acheive a true equivalency between different sized sensors and produce identical images we must consider the data collected at each individual pixel.
Simply put APS-C cameras have more dense sensors with more pixels, even with identical resolution. Because ISO is dependent on a given volume of light passing through a given area, if we increase the number of pixels in that area each pixel will have a stronger amplification level.
To illustrate this imagine a rain storm. On one part of the ground you have a set of square buckets that are 1" x 1" x 10", on another you have a set of buckets that are 2" x 2" x 10". After a given identical volume of rain, the 2x2x10 buckets will have 4 times more liquid in them than the 1x1x10 buckets. However four 1x1x10 buckets will have the same amount of rain in them as a single 2x2x10 bucket, as they will cover the same area (4 square inches).
Now lets imagine you have to create a given level of brightness from the amount of liquid in each bucket. Because the smaller buckets have a smaller cross section you have to multiply the amount of liquid in them for each smaller bucket to produce the same level of brightness as a larger bucket.
So on the 2x2x10 bucket, 20 cubic inches of rain might equal a luminance value of 128. For the same amount of rain fall each 1x1x10 bucket would only contain 5 cubic inches of rain, but if you wanted to have the same luminance value you would have to multiply the volume of light at each pixel by 4.
In this way, a sensor with denser pixels always has to multiply the volume of light to get a given luminance value more than a sensor that is less dense.
ISO is a function of the volume of light resulting in a certain exposure ie the amount of rain coming from the sky, resulting in a given exposure level. Say 5 inches of rain per hour resulting in a luminance value of 255, and 2.5 inches resulting in 128 in our example.
So to collect identical information ie number of photons at each individual pixel, with an APS-C and FF camera that have identical resolutions, we must use a different volumes of light hitting the sensor, specifically on APS-C we must have a higher volume of light, which results in a lower ISO to produce the same level of exposure. Just like we would have to use a greater flow of rain to collect the same amount of water in a smaller bucket compared to a bigger bucket.
Because light can simply be compressed and expanded at will by changing the beam path, which alters the volume of light in a given area (think magnifying glass), all that we have to do when converting an incoming image from FF to APS-C is focus the same image coming through an identical iris on a smaller area to capture the same exact information on an APS-C sensor as a full frame sensor. If you go through the geometry and the math of doing this equivalency something amazing happens though. At "35mm equivalent" ISO, focal length and aperture and the same shutter speed, on a full frame and APS-C sensor if you have a theoretically perfect body and lens system you would get the exact same photons
which came from the exact same sources from your subject landing in the same number and location in each individual coresponding pixel site and resulting in the exact same luminance values in the output for both cameras. With each camera having a different ISO setting, focal length, and aperture.
Identical photons from identical sources, landing in coresponding pixels, resulting in identical luminance values are what is important when desiring to create identical images. That is how 35mm equivalence works, and why it is important.
Discussing anything other than 35mm equivalent values for cameras is like saying I have a million dollars, and then failing to mention these are Zimbabwe dollars worth $20 not, American dollars.
Yes aperture ISO and focal length are fixed numbers, but so are monetary figures, and the most important thing even the most basic dealing of currency has is WHAT currency you're dealing with, and 99% of people require an "equivalent" frame of refference to understand foreign currency or need to do a conversion. Likewise with cameras, geometry (type of currency) is the most important thing when dealing with the performance of a camera system, and the first thing anyone needs to do is bring up a conversion to the local frame of reference, APS-C ,35mm, whatever.
So in other words theoretically a Crop set to:
#1. 17mm - f/2.8 - ISO 800 - 1/50th - with 1/4 flash
#2. 55mm - f/2.8 - ISO 800 - 1/50th - with 1/2 flash
Will produce a 100% identical image with no difference in exposure, lighting, depth of field, field of view or composition when compared to a full frame set to:
#1. 27mm - f/4.48 - ISO 2048 - 1/50th - with 1/4 flash
#2. 88mm - f/4.48 - ISO 2048 - 1/50th - with 1/2 flash
Literally no difference.
Now of course each lens will have it's own characteristics and each body will likewise have it's own, and the full frame bodies and lenses tend to produce higher quality images due to the fact that miaturizing the system has adverse side effects, but if both bodies and lenses were theoretically perfect and had the same resolution these settings would deliver the exact same images with completely identical pixels.
Hope that cleared things up.