Accurately comparing FF vs APS-C sensor performance? An open discussion.

Started Apr 8, 2013 | Discussions thread
harold1968 Veteran Member • Posts: 4,687
Re: the problem with this analysis

VirtualMirage wrote:

harold1968 wrote:

is that you have chosen to measure some specific variables

without commenting on how accurate or otherwise your measurements or sources are you miss a big thing about larger sensor vs smaller, that is the amount of light collected per unit area of lens glass

I'm fairly confident in saying that the source tested the cameras in the same manner, using the same constants.  Thus, any inaccuracies in their test methods would be the same across the board and would nullify each other out.  This means that comparing one to the other should be possible without an issue of one being more weighted than the other.

The scores/values don't reflect a lens being used, only direct measurements of the sensor.  But my adjustment, or shifting, of their values is based on how they would react with a lens attached and used in a way that matches my scenario.

its a well known fact that FF lenses don't have to work as hard as APS-c lenses. What does this mean ? It means that a lower quality FF lens will look sharper then a higher quality APS-C lens as imperfections in the glass are magnified more for smaller sensors (although conversely FF sensors require more glass and therefore higher tolerances).

Why are you comparing FF glass to APS-C glass?  We are not comparing glass here, we are comparing sensors.  Assume that both sensors are using FF glass.  I have an APS-C sensor but most of my lenses are FF.

As for the glass, from what I have seen, if the same piece of glass was used on both cameras usually the amount of light being transmitted through the lens to the sensor is a difference of .1 T-stops or less.  This doesn't make a large enough impact to show a discrepancy of 1, 2/3, 1/2, or even 1/3 stop that my results are showing.  So it could be then assumed that the difference it makes when on a FF sensor versus an APS-C sensor is negligible.

The biggest impact you will see between FF sensors and APS-C sensors is corner sharpness, distortion, and vignetting.  If you use a FF lens on an APS-C sensor you experience less of those issues since you are using the central part of the lens.  FF sensors use more of the glass and are more susceptible to it.

An APS-C sensor would need higher quality glass that offers a high central sharpness to get the most resolution out of the APS-C, but it benefits more of the overall image.  The FF sensor may offer higher central sharpness than the APS-C sensor with the same lens, offering a higher efficiency of sensor resolution to lens resolution, but gets sacrificed as you look further away from the center.  This is why you need higher quality glass for FF sensors over APS-C sensors.

To summarize, with a given lens , FF sensors will offer higher central sharpness but risk lower corner sharpness and distortion/vignetting while APS-C sensors might offer lower central sharpness but have a higher overall image sharpness and less distortion/vignetting.

there are other differences but putting technicals aside I find them large in the real world.

for example i still think the landmark Canon 5D produces sharper pictures then any APS-C camera made today (and the nikon D700 for that matter). i am talking about base ISO of course.

But we are not talking about sharpness.  We are talking about increased sensor noise when trying to match depths of fields across two different sized sensors while keeping the equivalent focal length and shutter speed the same.

i am comparing glass as lenses are under less stress when using FF over APS-C and that effects the whole picture

Its not a lens issue per se, if you use the same lens on a APS-C camera and a FF camera the differences are plain. If you look at the MTA charts on, for example, you will see that the same lens on a FF camera can reveal typicall up to 1/3 more detail. This is an incredibly large amount.

You are talking about light transmitted, but on DXOMARK this does not seem correlated to detail or colour depth. A fact which makes me think its missing some sizeable effect.

My experience suggests precisely the opposite of your claim, and if fact that FF sensors deliver bags more sharpness across the frame on average.

In term of you objective of matching depth of field and sensor noise, the smaller sensor always has an advantage when you are trying to keep more in focus at a larger aperture size. This is the fundamental tradeoff

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