What is an "equivalent" landscape APS-C lens? (also: exposure trapezoid???)

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Rado R Junior Member • Posts: 30
What is an "equivalent" landscape APS-C lens? (also: exposure trapezoid???)
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This thread is to continue an out-of-topic discussion that broke out in another thread. If you want to check it out, here it is:

https://www.dpreview.com/forums/thread/4532732

Context: A landscape photographer wants to buy a new mirrorless body. The dilemma is between a new APS-C or a full frame body. The criteria is:

  • Lightweight
  • Good dynamic range (they mentioned GRii, an APS-C body as being good)
  • Doesn't care about bokeh (not surprising for landscape photographer)
  • Great AF not important

Clearly, based in the above an APS-C sensor is good enough for the landscape photographer.

So, the following question arose:

What is the "equivalent" APS-C lens for a landscape photographer that doesn't care about bokeh, and is it smaller than its Full Frame counterpart?

Now, whether the lens is smaller or not really depends on the individual lenses chosen. So lets have some concrete options:

Full Frame lens: Canon EF 24-105mm f/4, no IS. Dimensions: 84 x 107 mm. Weight: 670g

Candidate A: Fuji 16-80mm f/4 OIS. Dimensions: 78.3 x 88.9 mm. Weight: 440g

Candidate B: Fuji 16-55 f/2.8, no IS. Dimensions: 83.3x106.0mm. Weight: 655g

Candidate A is clearly the smallest lens. The other 2 are pretty much equal. The focal range for both candidates are in the correct range to give the same field of view as the FF lens. But the FF lens is f/4! Does the APS-C have to compensate for the crop sensor? Is the equivalent landscape lens the f/4 or the f/2.8 APS-C lens?

Yes, if you want to shoot exactly the same photo, with the field of view and the same DoF, the equivalent lens would be the f/2.8. However, the landscape photographer does not need bokeh. On the contrary, landscape photography usually requires the entire frame to be in focus so shallow depth of field is not desirable.

I think that, given the constraint of landscape photography, the "equivalent" landscape lens is the f/4. This is because, if we don't care about DoF, one would be able to get exactly the same exposure with the f/4 full frame lens as the f/4 APS-C lens.

For example, if you shoot the same landscape with the same conditions with both the FF and APS-C bodies, if you don't care about DoF, both shots will require exactly the same settings (SS, Aperture, ISO) and equivalent focal length (e.g. 24mm for FF and 16 for APS-C).

However, some posters disagreed:

ecka84 wrote:

Unfortunately, you and many others (including your suggested youtuber) are terrible at math. Because FF 24-105F4 is equivalent to APS-C 16-70F2.6. Not F4.

...

ChelseaPhotographer wrote:

I think you will find that the one who is incorrect is you. An f/4 APS-C lens is equivalent to f/6.1 in full frame. So the FF 17-40 is more than 1 f-stop brighter than the Fuji. You are not really comparing apples to apples. And since the fuji lenses are a slower, you need longer shutter speeds...

...

ChelseaPhotographer wrote:

No, you are getting a lot more photons because of the larger area of the sensor, and no, if I take a photograph with an APS-C camera at f/2.8, in order to get the same image with a full frame camera I would have to close the aperture to f/4. And if instead I shoot at f/2.8 with the full frame, I will have to make my shutter speed 1 f-stop faster, otherwise the image will be 1 f-stop lighter than that of the APS-C camera... <sigh>

We had a lot of back and forth, but my answer boiled down to this:

Rado R wrote:

Yes, if you want to take the "exact same image" (which is actually impossible due to the different perspective of the different focal lengths) you will need to take into account aperture because of the depth of field.

... here comes the big BUT...

But, if you read the OP, we're actually talking about landscape photography here.

This is important! It is, in fact, the key point.

Have you ever done landscape photography? How important do you think shallow depth of field is for landscape photography?

It's not! Landscape photographers usually shoot stopped down. F8 or even higher.

So ... let's revise what "exact same image" means with respect to landscape photography. It means:

1) same field of view

2) same exposure.

Depth of field is irrelevant.

Now, answer this: how come there is no focal length in the exposure triangle? Do light meters care about how big is your sensor?

The answer is no, sensor size and focal lengths don't matter for exposure.

Now ... how about field of view?

Yes, that matters. A 24mm on FF will have the same field of view as a 16mm on 1.5x crop sensor.

Therefore, for LANDSCAPE photographer, the "exact same image" can be taken with:

- Full Frame at 24mm, f4, ISO 100, and 1/60

- Crop Frame at 16mm, f4, ISO 100, and 1/60

Then, I got some education:

ChelseaPhotographer wrote:

OMG. Pretty much everything you said is incorrect. I am not here to educate you and this is getting tiresome. But I will try one last time. Start with the last part. You say these two images will be the same...

- Full Frame at 24mm, f4, ISO 100, and 1/60

- Crop Frame at 16mm, f4, ISO 100, and 1/60

...but you are incorrect. The full frame image will be 1 f-stop brighter than the APS-C image. (I don't know if you have an APS-C and a full frame camera, but if you do, please just take the picture before you keep on going on...)

Lets keep the APS-C settings as a given

- Crop Frame at 16mm, f4, ISO 100, and 1/60

In order to get "the same" lightness in the image from the full frame camera you can do any of the following:

- Full Frame at 24mm, f/6.1, ISO 100, and 1/60

- Full Frame at 24mm, f4, ISO 50, and 1/60

- Full Frame at 24mm, f4, ISO 100, and 1/120

If you don't understand this, then you are simply not getting it. The change in aperture is not so that you have the same depth of field, although it is a byproduct. It is so that you halve the exposure of the full frame camera so that you have the same lightness.

And then more falsehoods:

ecka84 wrote:

Actually you both are missing the equivalent ISO part while reproducing the same image on different format cameras.

APS-C 16mm F4 ISO160 1/60 = FF 24mm F6.1 ISO400 1/60

FF 24mm F4 ISO100 1/60 = APS-C 16mm F2.6 ISO43 1/60

Unfortunately, you might not get the same brightness in practice, because Fuji cheats with ISO and you will get darker images.

After I got my education, I did some homework to prove that the exposure does not depend on focal length.

Experiment #1: Shoot the same field of view with the same camera at different focal lengths with all other settings the same. Of course, I had to crop the wider shots to match the others, simulating the effect of a crop sensor.

Rado R wrote:

Here is the test I outlined above (with the same camera - Canon 6D). My APS-C battery is still charging ... may be dead ... haven't used it in many years.

A: f/4, SS: 1m, ISO: 100, focal length: 105mm

BA: f/4, SS: 1m, ISO: 100, focal length: 50mm

A: f/4, SS: 1m, ISO: 100, focal length: 24mm

All images are taken with different focal lengths. Exposure settings are all the same. Edit was only for lens profile correction and crop.

Of course, this wasn't convincing:

ecka84 wrote:

You don't need a real camera for brick wall photography. This is not a test. This is a disgraceful attempt to glorify your own ignorance. You have proven nothing.

One would think that these photographers were talking about truly equivalent photos, including depth of field, but this was not the case at all. Seemingly, they are talking only about exposure:

ecka84 wrote:

You are the one talking about DoF. Just stop. Forget about DoF.

I posted a test with the same camera above that simulates sensor crop factor. Check it out.

Your tests don't prove anything.

I can take a FF camera with F5.6 lens and reproduce any image you take with your F4 lens on crop. That's the whole point. Crop optics are not smaller, period.

So I finally did the APS-C and FF comparison shots. Surely, this should be enough to convince them:

Rado R wrote:

OK. Here is the APS-C and FF test.

Full Frame: Canon 6D with 24-105 F4 @ 50mm

APS-C: Canon 450D with 24-105 F4 @ 30.9mm ...ish... ended up being 32mm

Edits: Lens profile correction and monochrome. Exported at 12MP to match 450D

6D @ 50mm

450D @ 32mm

6D @ 50mm -1/3 exposure in post

As you can see, the first 2 images are almost identical (again, not talking about DoF, IQ, resolution, etc ... just exposure and field of view). There is only a slight difference between the FF and the APS-C exposures which is likely due to either the base sensitivity of the different sensors or maybe my lighting slightly changed. Nevertheless, as you can see in the 3rd image, the difference is only about 1/3 stop and not a full stop as some here are implying.

So this is it ... exposure does not change for different crop factors. So there is no point equating an f/4 FF lens to an f/2.8 APS-C lens if all you care is exposure and not DoF. Therefore, for a landscape photography, where DoF is not important, the equivalent to 24-105 f4 FF lens is the 16-80 f4 APS-C lens, which is much smaller and lighter.

I thought this would be enough, but then I got the following response:

Cato1040 wrote:

Yea, equivalence is a tricky thing and there's a lot of misinformation out there that people end up believing that leads to off-topic debates like this.

...

To clarify with equivalence since there was some misinformation on both sides:

Full Frame: ISO: 100, SS: 1/100, Aperture: 4, Focal-length: 24mm

APS-C: ISO: 100, SS: 1/100, Aperture: 2.8, Focal-length: 16mm

With everything else being the same, the exposure will be the same, but that's because ISO is a made-up thing that makes it so that the exposure will be the same. Technically, the FF sensor will still have better IQ when it comes to light because it's still letting in more light. Whether that's significant will be up to the user.

I agree with the IQ point because of the larger photo pixels of the FF sensor, but exposure? Come on ... they don't even look at evidence when presented!

And then ... the following post made my day! I learned something new: the exposure trapezoid! But you can read it for yourself:

ChelseaPhotographer wrote:

The only reason why this is important is because if you are comparing across sensor sizes, then the "exposure triangle" is no longer true and instead becomes an "exposure trapezoid" with one of the sides being the sensor size. People don't realize this and think all cameras get the same amount of light, but that is not true.

Not sure it this will help the people who argued otherwise, but, when you switch a full-frame camera to APS-C mode, that central part of the image is receiving the same amount of light that an APS-C sensor would (if you are using the same lens), but if instead you use the full full-frame sensor to capture the same image as the APS-C camera, then the full-frame sensor is getting 133% more light than the APS-C camera.

If the OP is going to do night photography (or any type of low light photography), as they said they would, this becomes a very significant consideration.

The OP mentioned at some point that they were surprised how much more detail they could get from their GRII compared to their 6D. However, that is not because both sensors are capturing the same amount of light. That is simply because of how bad the technology and the dynamic range of the 6D is. If you compare the sensor of the GRII with a Nikon or a Sony full-frame, you will see that those sensors have even more detail in the shadows than the GRII, and in this case it is not because one sensor is technically better than the other; it is because the larger sensor is getting more light.

The first paragraph killed me!

But then, reading the second paragraph... yes!!!... this is the whole point! I don't know if it's cognitive dissonance or being lost in translation but this poster clearly knows that the light at the centre is the the same. Arguing that the exposure is different in the same breath is astonishing to me.

So there you have it. Congratulations on making it to the end.

What do you think? Am I off my rocker here or is the "equivalent" APS-C lens for a landscape photographer the same as a FF lens with the same aperture?

 Rado R's gear list:Rado R's gear list
Canon EOS 450D Canon EOS 6D Canon EF 50mm F1.4 USM Canon EF 70-200mm F4L USM Canon EF 100mm f/2.8 Macro USM +4 more
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