Discussion of 'Equivalence'

[David H. Hartman]

The perceived absolute amount of DoF is maybe dependent on the viewer. But different viewers would probably agree about changes in DoF, or about which of two images has more or less DoF. For background blur it's even easier to notice changes. Do you agree?

Yes I agree.

I have or had 20/80 - 20/50 vision uncorrected by my ophthalmologist gets me 20/15 corrected. I think 20/20 is pretty standard. 

Dave

[David H. Hartman]

I like the "look" but I can't afford FX. I want to know if I can get the same "look" with DX. How do I find out?

To get a similar look for a 105/2.5 on FX you'd need a 70~75/1.4. If you'd be satisfied with a rangefinder camera you are in luck. Leica makes a 75/1.4 ASP lens.

Dave

[David H. Hartman]

What are the trade-offs between using this zoomer on my 36MP D810 versus my 24MP D500 versus my 24MP D750??

I would not loose sleep over using the 200-500/5.6 on a D500. If you do you'll be using it because you have the D500 with you or you need the reach you get with the combination.

[The D750 has an AA filter so I would generally the D810. The D810 is also a larger camera so it would balance better with the 200-500/5.6. A grip would be useful for both the D500 and D810.]

Dave Hartman

Does a 300/4.5 have more reach on 35mm than a 300/9.0 on 5x7? Why are some so sensitive to the use of word "reach" with telephotos and DX v. FX.

I was short on time and added a little more above

[bclaff]

[/sup]What are the trade-offs between using this zoomer on my 36MP D810 versus my 24MP D500 versus my 24MP D750??

Not speaking to equivalence but assuming you are concerned with reach and would effectively be using the D810 and D750 in DX Crop mode it would appear the D750 would be a better choice than the D810.
The D750 in DX Crop would be very much like the D500 so I think that might come down to whether you would use the D750 without DX Crop mode for flexibility in later cropping.
Chart:

[David H. Hartman]

A hour ago I recalled that when making large prints from the same negative, Tri-X Pan (not Pro) from 35mm or 6x6/6x7 I'd need higher contrast printing paper for the same look as 8x10. As I recall this was mentioned in Ilford literature.

Dave

[Les Olson]

A hour ago I recalled that when making large prints from the same negative, Tri-X Pan (not Pro) from 35mm or 6x6/6x7 I'd need higher contrast printing paper for the same look as 8x10.

That is because very large prints are intended for longer viewing distances, and higher contrast enhances the impression of sharpness at longer viewing distances.  The 35mm or medium format images had less detail than the 8 x 10 so they needed more contrast to compensate.  Here is an example (from Allen & Triantaphillidou, Manual of Photography): the right hand image has more fine detail but lower contrast.  From normal reading distance the right hand image looks sharper, but from 2m the left hand image looks sharper.

[Les Olson]

I have or had 20/80 - 20/50 vision uncorrected by my ophthalmologist gets me 20/15 corrected. I think 20/20 is pretty standard. 

20/20 means you can see detail at 20 feet that a person with normal visual acuity can also see at 20 feet; 20/80 means they can see detail at 80 feet that you can only see at 20 feet and 20/15 means you can see at 20 feet what they have to be at 15 feet to see.  Outside the US 20/20 is called 6/6. 

[Les Olson]

Does a 300/4.5 have more reach on 35mm than a 300/9.0 on 5x7? Why are some so sensitive to the use of word "reach" with telephotos and DX v. FX.

Because it is misleading.  A 300mm on 35mm does not have more anything than a 300mm on 5 x 7.  It is a 300mm punto e basta.  If you use a 300mm on 5 x 7 then take a 35mm-sized piece out of the middle and print both the same size, did the lens mysteriously acquire more reach?  Then how does it make sense to say that 300mm has more reach just because the 35mm-sized piece of film is in a different camera?

[David H. Hartman]

Even looking at the larger print at the normal viewing distance of the smaller one the detail looked a bit flat. The print sizes were 8x10, 11x14 and 16x20.

I'm not doubting that extra contrast makes a larger print at a greater viewing distance look sharper. I was once at a print shop where they were printing strips for a highway billboard. They aren't really photographs. They are enhanced significantly as they must be to appear sharp at a distance. I'd guess the original was air bushed. This strips of image really looked garish at one's feet. A billboard is an extreme example.

Dave

[David H. Hartman]

I don't find the word "reach" misleading. A 300mm lens is a different lens on different formats. On DX (half frame) a 300mm lens is a super telephoto. On FX (full frame) a 300mm is less than "super." I read it in a magazine.

Dave

[Les Olson]

Here is a legitimate question for which the optics of Equivalence can be useful.

[/sup]What are the trade-offs between using this zoomer on my 36MP D810 versus my 24MP D500 versus my 24MP D750??

The way to think about it is the number of pixels on the subject.  If your subject just fills the frame with 500mm on the D500 you get 20MP on the subject, while with a D810 you get 15.4MP and with a D750 you get 10.3MP.   But if the subject just fills the frame with 200mm on the D500, equivalence says that 300mm on the D810 is the same, and that is wrong, because that 300mm on the D810 gives you 36MP on the subject instead of 20MP.   

The trade-off with using the 200-500 on the D500 vs the D810 is that you get more pixels on the subject with the D500 when you are using the longer end, but fewer pixels on the subject when you are using the shorter end.  The cross-over is between 300 and 350mm. 

Of course, the question is what you would do with all those pixels.  A 13 x 19 print at 300 dpi is about 10MP and at 360 dpi is 11.5MP, so the D750 limits size and/or output resolution at the long end.  One thing you could do with them, which equivalence forgot to mention, is adjust DoF by printing larger and/or at higher resolution.  The D810 and the D500 give you spare pixels to do that at both ends, but the D750 does not give you a lot of spare pixels at the longer end unless you are printing small.   

[Andrea B.]

Small correction:  At a fixed distance from a subject, Equivalence for lenses says that 200mm on the D500 and 300mm on the D810 both give the same diagonal angle of view (framing) and same perspective. Equivalence for lenses does not say the two settings are "the same" or that two photos resulting from those settings are "the same".

^{angle of view = 2*arctan[28.8/(2*200)] = 8.24°}

[Les Olson]

Small correction:  At a fixed distance from a subject, Equivalence says that 200mm on the D500 and 300mm on the D810 both give the same diagonal angle of view (framing) and same perspective. Equivalence does not say the two settings are "the same" or that two photos resulting from those settings are "the same".

Well, actually, "Equivalent photos, as opposed to "equal" photos, are photos that have the same perspective, framing, DOF, shutter speed, brightness, and display dimensions (http://www.josephjamesphotography.com/equivalence/), which is a good deal more than framing and perspective.  Of course, lots of things are not on the list, and in some cases - aesthetic considerations, eg - that is obviously appropriate.  The point is that output resolution is also not on the list, and the issue is whether that is justifiable. 

One way such things can be justified is as a simplifying assumption - ignoring friction in physics, eg.  Ignoring output resolution is not a simplifying assumption like assuming equal output size, or as standardising output resolution would be.  Output resolution must be ignored if display dimensions must be the same: it is impossible for a 36MP image and a 16MP image to have the same display dimensions and the same display resolution without extensive re-working.

Even standardising output resolution as a simplifying assumption would still need justifying: some simplifying assumptions can be justified - ignoring friction in physics, eg, but some cannot - ignoring informational asymmetry and irrational behaviour in economics, eg.  There has already been discussion about whether the simplifying assumption of the same output size can be justified; IMO it cannot, but opinions can vary.  But we have not been given any reason to justify ignoring the fact that output resolution can be varied independently of output size. That attribute of digital capture is one of its core characteristics, and one of the few ways in which it is genuinely an advance over film.  (Not the least bewildering aspect of the equivalence debate is being accused of being wedded to film-era concepts, then listening to the same people who make that accusation talk about how the FX image has to be "enlarged" less than the DX image to reach 8 x 10).

We are back to circularity: if you do not ignore output resolution equivalence collapses, therefore output resolution must be ignored. 

[simsurace]

Nothing speaks against defining a notion of equivalence which includes equal output resolution.
This would constitute a more restrictive notion of equivalence.
It requires down- or up-sampling in order to achieve the same output resolution from cameras with different native resolutions.
In fact, whenever the output is on a digital screen, this is implicit in the "equal display dimension" requirement, since the dimension and the resolution are linked.
For prints, the matter of output resolution and how it relates to the paper and printing method deserves separate studies.
Comparison of camera systems of very different native resolutions is fraught with difficulties that are best resolved on a case-by-case basis, IMHO.

There is no simplifying assumption in the definition of equivalence.
The definition is what it is, and can be changed depending on individual needs.
If one requires a stricter definition with more criteria, one is welcome to add them and specify the use of an alternative definition when communicating conclusions.
However, this narrows the scope of possible comparisons. When there are too many criteria, the comparison would be meaningless since by definition no differences between photographs will occur.
The list of criteria was chosen based on the idea that apples-to-apples comparison of camera performance needs to control for certain variables that would otherwise make a comparison difficult or affect the result in a way that completely changes the performance, making interpretation difficult.
Simplifications occur in the optical theory that describes the choice of shooting parameters in order to achieve equivalent shots.

[Les Olson]

When there are too many criteria, the comparison would be meaningless since by definition no differences between photographs will occur.
The list of criteria was chosen based on the idea that apples-to-apples comparison of camera performance needs to control for certain variables that would otherwise make a comparison difficult or affect the result in a way that completely changes the performance, making interpretation difficult.

Precisely!  Your assumption is still, however, that we have to make comparisons of camera performance, and therefore we have to do whatever is necessary to make comparisons feasible, and, for preference, easy.  It is not obvious to me that we have to make comparisons of sensor performance at all, especially when we are not making comparisons of, eg, AF performance and ergonomics, but we can leave that aside.  The idea that because including some variables, in this case output resolution, would make comparison difficult or impossible means that it is OK to arbitrarily exclude them cannot be right.  Excluding them may make the comparison possible, but it also makes it irrelevant. 

Down-sampling is not a solution, partly because there are different ways to do it and what the software does in any given case is not transparent, but above all because there is no reason to assume the photographer will do it.  As ever, we are back to circularity: the only reason you have advanced why we should accept the assumptions on which equivalence depends is that equivalence depends on them.