Downside of using FF lenses on crop bodies

I came across this YouTube video today:

https://www.youtube.co​m/watch?v=YDbUIfB5YUc

It explains why using a FF lens on a crop body will usually mean a decrease in IQ. Maybe I've missed something but I can't remember any thread on POTN about this before even though the way this guy explains it makes sense.

Comments?

teekay wrote in post #17924483
I came across this YouTube video today:

https://www.youtube.co​m/watch?v=YDbUIfB5YUc

It explains why using a FF lens on a crop body will usually mean a decrease in IQ. Maybe I've missed something but I can't remember any thread on POTN about this before even though the way this guy explains it makes sense.

Comments?

Usually the case that APS glass is newer and optimize for the higher pixel pitch. There are some exceptions, and sometimes no choice, but buying a bunch of wider EF glass to use on APS is a waste of money.

Interesting but there seems to be some info not quite right. Gonna have to dig into this one a bit.

It is true that the Image Circle's size has something to do with resolution, so that lenses designed to give a smaller image circle can have far greater resolution. For example, compare a lens for 4x5" sheetfilm vs. one made to make microfilm images of documents (newspapers).

the important observation is that the APCS lens focuses all the gathered light onto the smaller sensor which is of course not remotely true of a FF lens that is only projecting a fraction of the gathered light onto the smaller sensor

more light = more detail

wallstreetoneil wrote in post #17924512
the important observation is that the APCS lens focuses all the gathered light onto the smaller sensor which is of course not remotely true of a FF lens that is only projecting a fraction of the gathered light onto the smaller sensor

more light = more detail

If that were so, then the aperture of a lens would change when used on crop bodies. The FF lens will put just as much light on any given area of an APS-C sensor as it does on the same area of a FF sensor. That presenter's confusing the issue - the "aperture" effect he's talking about is only going to affect DoF, and that's actually influenced solely by the degree of enlargement you apply to the captured image; to get the same final print size from a crop body and from a FF body with the same lens at the same aperture setting, you'll have to enlarge the crop image more. It won't change the light-gathering capabilities of the lens, or the exposure. Try it - put, say, an 17-55 EF-S on a crop body, then put the 17-40 on the same body and take meter readings of both at f/4, 17 mm. You'll get the same exposure. That's because aperture is a "dimensionless" number; it measures the ratio of the lens entry pupil and the focal length. Light rays from any given point on your subject, and incident on any point on the lens entry pupil will all be brought to focus on the same point of the sensor.

That video's total BS.

i actually did this immediately after I wrote what I wrote - one of the problems with your assertion, and what I started to notice, was that each lense has a different t-stop - so you need equivalent T-stop lenses

i tested the 18-35 1.8 Sigma @ 35mm and @ F2 against the Canon 35mm F2 @ F2 and they metered 2/3rds of a stop differently - so how to even compare?

Mountain out of a molehill; the usefulness of those lenses is not negated by incredibly minor impact those differences will have in the real-life results.

The only downside of using EF glass on apsc bodies is cost. Everything else is Hooey-Hokum. I own a 6D and a collection of Canon, Sigma, Rokinon and Tamron lenses - from 12mm FE to 100-400L. I also have a 70D. I have sold all my ef-s lenses EXCEPT the 18-55 STM. Its primary purpose is as an incentive for someone to buy my 70D when I am done with it.

Lenses I use on my 70D regularly:
35IS (standard - low light), 70-200 f/4L IS USM (grandson's athletics), 85 1.8 (low light action).

Canon does not make comparable ef-s glass. Neither does anyone else. If EF glass were degrading my IQ, how would I ever know? There is nothing to compare it to.

Which way did you find the difference? According to Tony Northup, there'd be almost a full stop difference between the two, with the 35 f/2 being slower, without even taking T-stop into consideration. Given that the 18-35 has more glass, I'd also expect it to have a lower T-number.

Edited to add - your favourite camera is a 5DSR, per your sig. That's the same pixel density as a 7D - crop a shot to 1.6x and see if the exposure changes. By Tony's logic, it should.

Jon wrote in post #17924575
If that were so, then the aperture of a lens would change when used on crop bodies. The FF lens will put just as much light on any given area of an APS-C sensor as it does on the same area of a FF sensor. That presenter's confusing the issue - the "aperture" effect he's talking about is only going to affect DoF, and that's actually influenced solely by the degree of enlargement you apply to the captured image; to get the same final print size from a crop body and from a FF body with the same lens at the same aperture setting, you'll have to enlarge the crop image more. It won't change the light-gathering capabilities of the lens, or the exposure. Try it - put, say, an 17-55 EF-S on a crop body, then put the 17-40 on the same body and take meter readings of both at f/4, 17 mm. You'll get the same exposure. That's because aperture is a "dimensionless" number; it measures the ratio of the lens entry pupil and the focal length. Light rays from any given point on your subject, and incident on any point on the lens entry pupil will all be brought to focus on the same point of the sensor.

That video's total BS.

I don't remember him saying that a crop lens would give a different exposure time given a set aperture and ISO in the video. Are you are responding to wall streets incorrect deduction of the video or the video itself? If the video, which you end your post by slamming, then what specifically about the video is BS?

Jon wrote in post #17924614
Which way did you find the difference? According to Tony Northup, there'd be almost a full stop difference between the two, with the 35 f/2 being slower, without even taking T-stop into consideration. Given that the 18-35 has more glass, I'd also expect it to have a lower T-number.

I just did a much more controlled test.

When I knocked out other lights (the TV that was on and in the corner) the 7D2 actually metered both lenses the same - so I correct my earlier assertion.

I'm going to do a bunch of tests next week on this issue.

MM!
If what Tony N says is true why do the images from my 7D2 with my 16-35 F4 L IS and 24-70 F2.8 L V2 look so much better than all the APSC lenses that I have tried on that camera? Obviously I am doing something wrong!
I bought these lenses for my 1DX but they work exceptionally well on my 7D2.

johnf3f wrote in post #17924800
MM!
If what Tony N says is true why do the images from my 7D2 with my 16-35 F4 L IS and 24-70 F2.8 L V2 look so much better than all the APSC lenses that I have tried on that camera?

You have best case scenario with relatively new generation glass. I don't think there would be ANY discernable difference in IQ.
I kinda' think this is a bunch of hooey as well.

johnf3f wrote in post #17924800
MM!
If what Tony N says is true why do the images from my 7D2 with my 16-35 F4 L IS and 24-70 F2.8 L V2 look so much better than all the APSC lenses that I have tried on that camera? Obviously I am doing something wrong!
I bought these lenses for my 1DX but they work exceptionally well on my 7D2.

Two things:

1) don't confuse 'look so much better', which has a lot to do with rendering and colours, with resolving power / potential mega pixels
2) you did not mention your apsc lens and Tony did not say it is true of every lens - what he was pointing out was that you have to test actual resolving power, as DXO has done, to know the resolving power of each lens on each camera

I am going to re-watch the video again and do some testing, but part of what he said I think came over in a slightly confusing manner. For example, let's take the 5D3, the 5DSR, the 7D2 and the 35mm L II and look at what DXO says about Sharpness and P-Mpix

5D3 - Sharpness = 18 P-Mpix
5DSR - Sharpness = 37 P-Mpix
7D2 - Sharpness = 14 P-Mpix

What Tony said, and may be confusing, is that a FF Lens on a Crop body might not be the best idea. I think he maybe made a mistake or has possibly misinterpreted DXO data - or I am incorrect. But if you look at the above data, it is interesting if you do a calculation on the 5DSR 37 P-Mpix and convert it into the size of a Crop sensor.

If you take a Canon Full Frame sensor resolving ability / sharpness and divide it 1.6 and then divide it by 1.6 again, this will tell you the resolving ability of that full frame sensor if it was converted into an APSC sized sensor. So, given that we know that the 5DSR is a glorified Full frame sized 7D2 sensor, if we divide the 5DSR's 37 P-Mpix by 1.6 and by 1.6 again (37 / (1.6^2) ) = 14.4 which is exactly what the 7D2 Sensor is resolving (coincidence? - don't think so). Tony talks about this in another video and mentions it in this video and that is if you are going to crop the image from a full frame sensor, to make it close to a Crop size, you very likely should have used a Crop to get better resolving ability. In the case that I just showed, we already know that a 5DSR basically has a 7D2 sensor imbedded in it and DXO's number seem to show this. If you take the same 5D3 sensor and divide by 1.6^2 you get approx 7 P-Mpix - so this would say, that you should use the 7D2 with the 35mm L II if you need to crop the 5D3 to a crop size (which everyone already knows).

Interesting that the crop lens that Tony mentions, the Sigma 18-35 F1.8, that he mentions as basically the sharpest crop sensor lens you can buy, that DXO states the P-Mpix = 13 P-Mpix on the 7D2 - which is slightly less that Canon's new latest and greatest, sharpness out of this world, 35L II at 14 P-Mpix on the 7D2 as mentioned above.

I think there maybe something in what Tony is saying but I have to figure it out because I think it could be partly wrong.