DoF width on a macro lens at various f stops?

swag72

swag72

Suspended / Banned
Messages
7,969
Name
Sara
Edit My Images
Yes
I don't know if there is really an answer to this one, but thought I'd give it a try.

I have a 100mm macro lens that gets an outing every so often. I know that with macro you get a very small DoF. I wondered whether there is a scale for each decrease in aperture, the DoF increases by a set amount? Similar to shutter speed and aperture perhaps?

Thought this may help in my macro photography in getting the right DoF.
 
Hi Sara,

Not sure if this is what you are looking for but this is an online DoF calculator.

http://dofmaster.com/dofjs.html

You simply enter the Camera type, focal length, aperture & distance from the subject and it tells you the Depth of field.

I think there are other sites where you can print off a chart based on the lens details you entered rather than needing to use an online calc.
 
Is a macro lens not different? We keep talking about the narrow DoF with the macro lens's, so in my head it would be different to other lens's.
 
Hmm, you might be right. Hopefully someone with more knowledge than me will chip in soon. Sorry :)
 
The difference with a macro lens is closer minimum focus distance so you can get closer. Its this closer distance that causes the DoF to decrease. The apperture value alters the proportion of DoF, not an absolute distance

eg suppose at f/1.4 you get a DoF of about 5% of focal distance, working at 10m gives you 50cm depth in focus but working at 10cm gives you only 5mm depth in focus

I know this probably isn't the clearest explanation but if you look at the DoF calculator linked above and try a few examples it might help
 
Can that easily be converted to a rough and ready mm guide? Tried the calculator above - Got confused - Doesn't take much!!

For example - at f2.8 xx mm DoF, at f4.0 xx mm DoF and so on? This would help me when assessing what aperture to use to the small subjects:thumbs:
 
Forgive me if I'm missing your point, but I'd say the best solution would be trial and error. Our brains are quick learners and one can get a feel for the best aperture to use just through experience. Take 10 macro shots and you'll already build up and instinct for which aperture setting is best to use.

Tell me I'm being too idealistic, but I feel tables of numbers take the art out of togging.
 
Macmike makes a good point. Take a few shots at different apertures and decide what looks best.

While defined numbers and distances do have their place, what looks good for one subject might not work for another.
 
If you have the paperwork that came with the lens when it was new, there may be a chart in the manual that tells you the DoF at assorted distances and apertures. IIRC from the glance I gave the sheet that came with my Tamron 90mm, the DoF can be measured in tenths of a mm when wide open at the closed focus distance (but it's in the box up in the loft so I'm not going to check now!).

At smaller apertures (possibly by f/11) you'll start running into diffraction softness but the extra DoF from the smaller apertures is often worth it.
 
As has been suggested, a bit of experimentation is best with macro. Try shooting a ruler at various distances and f/numbers and note the results.

The main problem with tables like the DOF Master is that you can't really put the focusing distance in accurately. What exactly is the distance from camera to subject? Where do you measure from? Front of the lens, which often moves out as you focus closer, depending on brand? From the image plane? Or from the 'centre of the lens' which I think DOF Master uses (and this often moves, too).

Calculators just don't work for macro :(

Richard.
 
You cannot use DOF tables for macro it is a different calculation. It`s simple really. All you have to know is the aperture and the C of C, then decide what magnification you require.1:1 down to 1:10
 
oldfella said:
You cannot use DOF tables for macro it is a different calculation. It`s simple really. All you have to know is the aperture and the C of C, then decide what magnification you require.1:1 down to 1:10
Click to expand...

That's just plain wrong, it doesn't change because its a macro lens, it would change is using extension tubes but that's not the case.

I reitterate, the only difference is focal distance and as Hoppy pointed out that is hard to guess accurately
 
From Here

Depth-of-field data for EF100mm f/2.8 Macro USM

canone8.gif


Example:

At a focusing distance of 0.31m (where you get a magnification of 1:1 with this lens) and at f/16 the depth of field covers (approximately!) the range from 0.311m to 0.313m. That means you have a depth of field of

FD - ND = 0.313m - 0.311m = 0.002m = 2mm.
 
Uneducated_Rick said:
That's just plain wrong, it doesn't change because its a macro lens, it would change is using extension tubes but that's not the case.

I reitterate, the only difference is focal distance and as Hoppy pointed out that is hard to guess accurately
Click to expand...

Not wrong exactly. I think what OldFella is saying is that since the focusing distance is hard to calculate then you need to subsitute the magnifciation ratio, which you simply read off the lens barrel.

I don't think you can do that with the DOFM calculator, but I just googled it and came up with this link:
http://www.robertseber.com/calculators/MacroDOFCalculator.html
I've not seen it before, and no time to check it now, but it looks well handy. Anyone care to road test it? ;)

And by the way, referring to another thread on macro and how the effective f/number reduces two stops at 1:1 etc, you still need to input the actual f/number for DOF calcs.

However, I still think that a few snaps of a ruler will give a very good visual idea of what's happening. That Canon table for example while being useful for owners of that particular lens (it takes measurements from the front element, yes?) suggests 2mm depth of field in the example given, but what does 2mm look like in practise? Do you know what I mean? I think macro is so different in many ways that a visual idnicator is better than a strictly technically correct one.

And anyway, since you usually stop down to get as much depth as you possibly can, maybe it doesn't matter much in practise :lol:

Richard.
 
I never said it was the lens that changed the calc. looking at that chart it seems strange, you cannot choose the magnification without doing another calculation. Also the DOF for macro does not use the 1/3 in front 2/3 at rear, it is equal 1/2y 1/2y
 
Good article here:

http://xoomer.alice.it/ripolini/Close_up.pdf

It goes into some detail with the maths but there's also a simple chart show DoF at various magnifications/f-stops which are the same for all macro lenses - DoF is based on magnification and aperture.
 
But surely magnification is a factor of focal length (of the lens) and focal distance?

Having not owned a macro lens I do not know if they have a 'magnification scale' on them, whereas I am sure some have a focus scale
 
Yes they do. Set the lens to your desired magnification then move the camera to focus.
 
Uneducated_Rick said:
But surely magnification is a factor of focal length (of the lens) and focal distance?

Having not owned a macro lens I do not know if they have a 'magnification scale' on them, whereas I am sure some have a focus scale
Click to expand...

Yes, but think about it.

A 50mm macro gives 1:1 magnification
A 100mm macro gives 1:1 magnification

Different focal lengths but so too is the focal distance, it's shorter on the 50mm. Both end up with 1:1 magnification. DoF calculators actually work out the magnification based on length and distance.

Try a couple of simple examples.

100mm lens at 5m
200mm lens at 10m

DoF will be the same for a given f/number.
 
If anybody is interested the calc for DOF in macro is 2*A *CC *( M+1)/ M^2.
For example 2 * 22 * (.5+1)/ .5^2 = 6.6 mm. .025mm is the optmum C of C says Zeiss. This gives a 1:2 shot. So it is 3.3 mm in front of and 3.3 mm behind. "Stop crying at the back there"
 
oldfella said:
"Stop crying at the back there"
Click to expand...

"Please Miss, what do I do about the crop factor?" Sob.

Which reminds me to mention, when you use magnification ratios to determine depth of field it becomes obvious that depth of field reduces dramatically as magnification increases.

So, if you want maximum depth of field, you can use this fact to your advantatge by turning it round. In other words, if you back off a bit so that the subject doesn't quite fill the frame, then do a bit of digital zoom in post and crop the image to the right size, you will get usefully greater depth of field.

And for those at the back of the class, no need to do the maths, just take a snap of that ruler again. If you can live with the slight reduction in quality due to the extra enlargement, it really makes quite a big difference.

To prove the point, I've just done it. I just photographed a measuring tape and at a magfication ratio of 1:1 depth of field looked to be about 5mm. Moving back and taking the same shot this time at 1:2 and enlarging it on the LCD screen to the same size, depth of field has gone up to 10mm or so. 100% increase :)

Both pics were at f/8, but it's also handy to remember that the effective f/number at 1:1 is two stops less, ie f/16, while with the 1:2 shot it was reduced only one stop, to f/11.

All in all, this would seem to be a pretty good trade off when you need as much depth of field as possible.

Richard.
 
You must log in or register to reply here.
Back
Top