Is this a way to approximately measure the magnification?

All Ex · Post by **All Ex** » Wed Sep 02, 2015 11:38 am

In order to, for the first time, get in touch with my stack shot, I performed a focus stacking with my macro lens and my extension tubes. I thought that it would be interesting to measure the magnification with the aid of Photoshop. Firstly I simple photographed my objects (two small sea shells) by a ruler, and then, without making any dimension adjustments on the result of the stack, I placed the two magnified shells next to the initial photography, with the use of guides it gave me something like that:

Estimating (approximately) the magnification is nearly to X 1.7 ( in fact X1.68 )
Has this way any reality in this calculation?

--Moved from archive forum and slightly edited for clarity, adminCS

Pau · Post by **Pau** » Wed Sep 02, 2015 3:05 pm

When you take a picture of a rule like this one, it's easy knowing your camera sensor dimensions:

magnification = sensor_width / field_width

rjlittlefield · Post by **rjlittlefield** » Wed Sep 02, 2015 4:16 pm

All Ex, be aware that the word "magnification" is used in several different ways.

If you have a print in hand, then "magnification" can very reasonably mean print_width / sensor_width. This is the number you'll often find used in magazines.

If you're looking at a screen, then by analogy "magnification" could also mean image_width / sensor_width. This one is not so reasonable, because the same image could be displayed on a cell phone or a large-screen monitor -- of course with much different magnifications. It's only useful when you're talking with somebody who is looking at the same screen you are.

Here at photomacrography.net, we normally talk about "optical magnification", which refers to the size ratio between subject and optical image on sensor. That one goes with the formula that Pau gave, and that's also the one that you need to use DOF tables such HERE.

When dealing with microscopes, there are other definitions of "magnification", some of which can get very confusing.

So now the important question: why do you care? For what purpose are you going to use the number?

--Rik

rjlittlefield · Post by **rjlittlefield** » Wed Sep 02, 2015 7:55 pm

By the way, I have no idea how you got a magnification of 1.68X for this image. From your ruler, the field width looks like 72 mm. An APS-C sensor is around 23 mm wide , so that would be about 0.32X optical magnification (=23/72). With a full frame sensor, it would be about 0.5X (=36/2). The image on my screen is about 212 mm wide, so that would be about 2.9X (=212/72).

What is that you measured and/or calculated to get 1.68X?

--Rik

Bushman.K · Post by **Bushman.K** » Wed Sep 02, 2015 7:55 pm

I know that it's a kind of philosophical thing, but some people (including me) prefer other measurements:
- width or diagonal of field in linear units (for example, "5..8 mm")
- average size of pixel (that's width of field divided by horizontal resolution of your sensor).

Being aware of width of field, it's easy to figure out, does particular object fit.
Being aware of pixel size, it's easy to create scale, if you need, and you can always calculate size of scene if photo is cropped or resized by known multiplier.

All Ex · Post by **All Ex** » Thu Sep 03, 2015 12:11 am

I was aware of the classical determination of the magnification (magnification = sensor_width / field_width) that Pau wrote. The thing is that dividing the size that the magnified cell has on the photo (23 mm) by the actual size of it (12 mm) it gave me a factor of nearly 2X magnification, that tricked me (by the way, I don't recall the way that gave me that 1.68 magnification). According to the tables of magnification in ZS page I have about 1.4 magnification since I use a FF camera (am I reading the write way the table ?).
About the answer Bushman.K gave me, I `ll have to do some more research, and I will do it some time in the close future. For the moment I`m occupied with the construction of a small chilling box that will act as a mini studio for my macro shots.

Thank you all for your time and your answers.

rjlittlefield · Post by **rjlittlefield** » Thu Sep 03, 2015 12:19 am

Is that scale with the shells marked in centimeters?

If it is, then your field width is 72 mm. With that, on full frame, the magnification = 36 / 72 = 0.5 .

The closest line of Table 1 at http://www.zerenesystems.com/cms/stacke ... romicrodof is the one for 70 mm field width. For that, the magnification on full-frame is the last number in the row, 0.51 .

--Rik

All Ex · Post by **All Ex** » Thu Sep 03, 2015 12:49 am

Now I understood what you and Pau meant by field width and how that apply on the table. Thank you very much Rik.