The Difficulty in Explaining Equivalence

I'm fully conversant with equivalence. What I need explained is how a member becomes "@Deleted" here. I didn't know that was a thing.

Facts! But, the main takeaway from Equivalence in practical terms for most people is the following:

1) The sensor size does not play a direct role in the noisiness of a photo.
2) The distance from the subject determines the perspective -- the focal length determines the framing of the scene.
3) In lower light, shallow DOF, motion blur, and noise are all related.
4) Using a TC and/or cropping has the same effect on the photo as using a smaller sensor.

Slap on an RF 28 / 2.8 pancake, stop way down (f/8?) and use software to fake the [background] blur (and/or stack exposures) but have more resolution, less noise, and more DR otherwise. But, considerably less convenient than a smartphone, for sure, and most people probably wouldn't care about any IQ differences.

Not if the steel is spread out on a large thin sheet! 😁

What did I miss?!

If the sheet hits your head edge-on (or corner-on if it has sharp corners), there could be pain involved.

please explain further.

It's the lens, not the sensor. The aperture diameter of the lens for the left photo was 75 / 18.15 = 4x as large, resulting in 16x the area, hence 16x the light projected on the sensor for the same scene and exposure time, thus 1/4 as noisy (if the sensors record the same proportion of light projected on them and add in the same amount of electronic noise, both of which are likely more or less the case).

My “basic misunderstanding” is why anyone would want to take an “equivalent” photograph. I always strive to take the best photo I can, not an “equivalent” photograph.

I see the value in helping someone choose equipment, but I don’t see any value in actual photography. I suppose I could use the math if I wanted to take my camera to Yosemite Park and stand exactly where Ansel Adams stood with his 8x10, and shoot what he shot. So I’d know what focal length and aperture could reproduce the photo on a 35mm sensor.

In some cases, the 'best photo you can' take will be determined by an 'ideal' shutter speed, an 'ideal' DOF, and an 'ideal' framing. If you own more than one format, and if more than one of your cameras can provide those same ideal parameters, you can consider any of them for the job. There will of course be other parameters such as sensor resolution, lens quality, bulk and weight, etc. that influence the decision. But understanding which equivalent photographs can or can't be produced by which combination of gear is useful to anyone who owns and uses more than one format.

If you don't own or use more than one format, equivalence is probably of little interest to you.

the problem with your maths is that the a7iv can recover 4 stops of shadow detail effortlessly.

I don't know where you got that from, but the point of equivalence as a concept is to help you predict image characteristics with various gear, relative to each other, independent of sensor size, not to dictate to you what gear or parameters to use.

dont show GB 🫣 14 years ago
heavy crop where i was second shooter for a grad shoot. i was using my K7 but wanted a comparrision image with my zx1.

I’ve been a photographer for nearly five decades. From that experience, I am certain that I do not know everything, and I don’t pretend to, and I am certain that the way I do things is not how everybody else should do them.

I can see the usefulness if you’re comparing APS-C camera A to full frame Camera B. But that’s really an academic pursuit, or maybe something for a reviewer. But, yes, I truly do understand the point of it, it’s the practical application that snags me up.

Let’s say I’m shooting a wedding and my full-frame camera breaks, and I grab my APS-C backup camera…. I would definitely want to have my equivalencies down. To explain: I study my equipment thoroughly and I have taken many shots at all of the focal length/shutter/aperture combinations to get a solid feel of what result I’ll get. I use 85mm at f/1.8 a lot, but if I need two heads in the shot, I know that I need to close down to at least f/5.6 or f/8. BUT… if I pull out my APS-C backup body and keep using my full frame lenses (which is what I would do), now all of my equations would need to be converted. First I would use shorter focal lengths across the board, and that means correspondingly larger apertures, if possible, if I want the same depth of field. Which would probably drive me so crazy that I’d just buy a full frame backup body. Better yet, I don’t shoot weddings. ;-)

But I do see how someone in a situation such as that would need to be prepared with a full understanding of equivalent focal length/apertures if they’d need to swap in an APS-C camera.

I feel like I have grasped the basics of equivalence but sometimes the statements posted in the DPR m4/3 forum are seemingly made with such authority as to make me question my sanity! 😟

I've appreciate GB's incursions to set things straight although the last time that happened, the moderator cracked the sh!ts as they don't really want anyone to speak about facts.

Anyway, here is an example of a recent thread that got locked: www.dpreview.com/forums/post/67621739

The OP posted a thread titled "*But equivalent FF lenses aren't any bigger than MFT lenses*", a statement with which he disagreed and would prove it to be wrong by posting a series of camerasize screenshots showing various m4/3 lenses that were supposedly equivalent to various FF lenses.

I felt that he was cherry picking the lenses to show some extreme differences, so I posted a few camerasize screenshots that I thought were a more fair comparison: the S5II with Sigma 35mm/f2 vs the OM-1 with MZ 17mm/f1.2 and the S5II with 70-300mm/f4.5-5.6 vs OM-1 with MZ 40-150mm/f2.8.
(Link to my post: www.dpreview.com/forums/post/67622134).

This resulted in a debate with another forum member named EZGritz. The thread was eventually locked (par for the course) so he then sent me this PM. And don't worry, it was just a continuation of the debate, nothing personal or private about the post.

He said:
"Too bad the thread is locked. What you said about aperture is absolute nonsense. An f/2.8 lens is f/2.8 and f/5.6 is f/5.6 no matter what sensor you mount it to. The lens aperture has nothing to do with the sensor. Mount it on different size sensors and the camera picks the same shutter speed for proper exposure. Try it and see."

I responded:
It is not nonsense at all. For example, an MFT lens that has a focal length of 25mm provides the same angle of view as a FF lens that has a focal length of 50mm. Hopefully you agree on this point?

The effective aperture for a lens (the diameter of the opening though which light is projected) is calculated as the focal length divided by the f-stop.
For MFT: 25mm ÷ 2.8 = 9mm aperture
For FF: 50mm ÷ 2.8 = 18mm aperture

If both cameras are set to f2.8, the FF camera will get more total light because the physical aperture is double the size. Your comment that f2.8 is f2.8 and f5.6 is f5.6 on different format cameras is dead wrong. On MFT f2.8 is the same as f5.6 on FF. The difference is x2. For APS-C, the difference is x1.5.

His next response was this:
"You don't understand the fundamentals of photography, and seem to have no idea how aperture is calculated. Lens aperture is calculated from the ratio between the size of the front element and the size of the rear element. It has nothing at all to do with the focal length or the sensor size or the angle of view. You are applying the difference between the diagonal measurement of the sensors and thinking it changes the aperture of the lens. It does not. The sensor size has nothing to do with the lens aperture. A larger aperture passes more light through the lens. f/5.6 is two stops slower than f/2.8 no matter what sensor size body you mount it on. Light is spread across the sensor. If the sensor is bigger it needs more light to make the same exposure. That makes it a zero-sum measurement. It's the amount of light that falls on a specific area that determines the amount of exposure. Just pick up any body with an f/5.6 lens and see it's less exposed than any body with an f/2.8 lens. F-stop is not the physical size of the aperture. It's the ratio between the front element and the back element in the hole made by the aperture blades. f/2.8 is the same ratio - f/2.8 on any lens no matter what the sensor or lens size is. It has nothing to do with the sensor, The DOF, not the amount of light that passes through is different. If you were right aperture would be measuered as diameter in millimeters, not in f-stops. You are wrong."

I was getting out of my depth at this point so I referred him to GB's equivalence essay. To which he responded:

"The DOF, FOV, and FL are different. F-Stop passes the same amount of light at any given spot on the sensor and ISO is the same for a given shutter speed and exposure. Point both formats at a subject at the same time and you will see the shutter speed is the same for the same exposure. Diameter of the aperture is different within the same format because the lens diameter varies in size. The F-stop can be different. In the OLY/OMS world there are three 40-150 lenses. f4-5.6, f/4 and f/2.8 constant. They are different sizes but the ratio between the front element and the aperture change the F-Stop value not the diameter of the aperture. The number of elements, quality of the glass this ratio determine how much light psses through the lens. The other formulas are invented to sell the concept that the small sensor is inferior in low light. It accomplished that but it is wrong."

"Joe James is discussing what system has an advantage for sharpness, noise, DOF depending on what is more important for a given image. He says M43 delivers more noise for the same exposure. Unless something like high resolution mode is used because it's noise-eliminating when the images are merged. He says the larger system delivers total light but he does not say it changes the exposure which is what you are missing. The aperture value delivers the same ISO and shutter speed for the same exposure. The physically bigger system delivers more total light but not more light to each pixel. Pixel size does not matter and why SAF on M43 will focus in lower light than FF will can with PDAF. You can see that in a video. FF will fail to find focus in low light when M43 with SAF can find focus and not in a theory, in the field. I have to look for that video. I didn't bookmark it. It was made by a professional photographer who shoots Nikon DSLRs and M43."

"You are comparing a variable aperture lens to a constant aperture lens to make the point that the size weight is equal. Not if you match the lens apertures. Use an f/2.8 FF lens of the same FL and see how they compare. Or compare the 40-150 f/4 or f/4-5.6, or the f/4-5.6 14-150 if you want to use variable aperture lenses and see how the size/weight and cost changes."

I've not responded further because [a] I am out of my depth and [b] I suspect that he has a bunch of things tangled up and I don't know how to untangle them (because of [a]).

Paging @GreatBustard.

this is what happens when you change sensor size from FF to apsc using the same lens