Does it make sense to talk about 35 mm eq. magnification?

[sagarmatha]

My picture below of my "fellow" Cicindela campestris was taken with my FZ50 at full zoom and Raynox 250. This is the closest I can get. The calculated magnification is 0.70. But that says nothing to me. Also my focal length 88 mm says nothing. But if I convert it to 35 mm eq. then everything makes sense: 88 mm -> 435 mm, 0.70 -> 3.5. Measuring his eye to about 10 % of the width together with the figure 3.5 gives me the clue that the width of his eye is about 36/10/3.5=1 mm.
Comments?

[rjlittlefield]

With respect to magnification only, it makes perfect sense in this case to work in 35 mm equivalences.

With respect to DOF, even this case has to be handled carefully. At f/11, the 88 mm lens will have an entrance pupil that is 8 mm wide, while the (imaginary) 435 mm lens would have an entrance pupil that is 40 mm wide. The latter would provide only 1/5 as much DOF.

The situation gets even more tenuous when you are focusing by extension and not by closeup lenses. Suppose that on the 35 mm camera you have a 100 mm lens operating at 1:1, so that the subject width is 36 mm. The lens-to-subject distance will be roughly 200 mm. If you simply apply the 5:1 ratio of sensor sizes, you will conclude that the "equivalent" lens for the smaller sensor camera is 100/5 = 20 mm. To image the same 36-mm-wide subject, the smaller sensor & shorter lens will have to operate at a magnification of 0.2X, which will produce a lens-to-subject distance of about 120 mm. Obviously 120 mm is not the same as 200 mm, so the lens that is "35 mm equivalent" does not result in the same shooting conditions. The actual equivalent, in this case, would be a 33.3 mm lens, which would image the same size subject at the same distance on the smaller sensor.

On a related note, I am curious.

As you say, 36/10/3.5 = 1 mm, using the 35 mm equivalent sensor size and magnification.

But of course 7.18/10/0.70 = 1 mm, plugging in the actual sensor size and magnification.

Why does the first "make sense", while the second "says nothing"?

--Rik

[sagarmatha]

Yes it makes sense to me because I know my equipment. But if I were to go to a macro meeting where all people shoot with cameras with different sensor sizes it would be a mess to look at different photos if they all labeled them with the actual magnification (0.7) instead of the 35 mm equivalent (3.5).

[rjlittlefield]

Agreed.

So why specify magnification at all?

The image shows an area that is 10 mm wide.

Nothing more needs to be said.

Let each photographer figure out how to do that with his own equipment.

--Rik

[sagarmatha]

Agreed. But magnification as a measure sits in my spine. Not easy to take away.

[rjlittlefield]

Maybe it's time to learn a new language. Teach it to your friends, even.

Talking in terms of field width saves many problems.

--Rik

[elf]

Yes it makes sense to me because I know my equipment. But if I were to go to a macro meeting where all people shoot with cameras with different sensor sizes it would be a mess to look at different photos if they all labeled them with the actual magnification (0.7) instead of the 35 mm equivalent (3.5).

Actually that would only make sense to someone that shoots 35mm. I would have to figure out how to translate it back to the sensor size I'm familiar with. Common terminology is to use magnification on the sensor/film. If you look at the labels on macro lens, none of them will specify sensor/film size. When you're looking at prints (or any other output device) then the magnification level will be significantly different than what it was on the sensor assuming you're not looking at 8x10 contact prints.

[rjlittlefield]

Talking in 35 mm equivalent terms produces other strange results.

Photographer: "Do you like this? It's 1.6X and uncropped."

Viewer: "It's gorgeous! What lens did you use?"

Photographer: "That's done with a Sigma 105 DG. It goes all the way from inifinity focus to 1:1 without any attachments."

Viewer: "Sounds nice. So then you attached some rings or an added closeup lens?"

Photographer. "No."

Viewer: "?????"

What happened, of course, was that the photographer simply stuck that 1:1 lens on an APS-C format DSLR. The smaller sensor size maps into a smaller subject frame, 23 mm wide instead of 36 mm wide.

Now, 23 mm wide would be 1.6X if it had been shot in 35 mm format.

But since it wasn't, using the 35-mm equivalent number actually interferes with communication, rather than facilitating it.

--Rik

[sagarmatha]

I think we stop this thread right now