Quick Macro by Tubes Question

When trying to match a lens focal length to different size macro tubes, do you factor in the 1.6 crop factor when using aps-c cameras?

The crop factor is -never- used for anything except for comparing the field of view between different formats. That is why there are so many of us on this forum who are so adamant that people should just forget about all the crop factor BS.

Agreed, just buy a set of tubes and go play with them.

Thanks. I just got my tubes and I am having a great time testing them out on my 7D and 5D. I was just wondering if there would be any difference between the two and macro ratios.
Just to verify, will my 85mm(136mm on crop) 1.4 will have the same effect on both bodies with the same length tubes on the lens?

ChadAndreo wrote in post #13106101
Thanks. I just got my tubes and I am having a great time testing them out on my 7D and 5D. I was just wondering if there would be any difference between the two and macro ratios.
Just to verify, will my 85mm(136mm on crop) 1.4 will have the same effect on both bodies with the same length tubes on the lens?

The tubes simply increase the lens close focusing ability. That's the same as saying it increases potential magnification. Of course you lose the ability to focus to infinity while the lens is mounted on the tube.

Those factors will be the same, whether on crop or FF. Magnification, close focus distance remain the same.

You will still see the angle of view difference between the cameras, just the same as you do using the lens on the two different cameras.

Two things you might find different:

1. On some lenses, particularly those that aren't flat field design, you can see some softness in the corners and/or vignetting when using the tubes. This isn't necessarily a bad thing... You often can use it to good advantage in your images. Or, in the case of vignetting, you can adjust it away in post processing or perhaps with Peripheral Illumination Correction feature on cameras that have it.

Because the FF camera uses more of the image circle from any lens, you will likely see these effects stronger on that camera, a little less on the crop camera.

2. Often when shooting close-ups/macro, you get very shallow DOF. It might be possible to stop down a little more with FF than with crop, before diffraction causes problems. Rule of thumb I use with crop is f8, f11 in a pinch. I try not to go beyond that. On FF, I'll use f11, f16 and possibly even f22 (though I try not to use it).

This is oversimplifying it, there are some differences in the effects of diffraction depending upon the density of the pixels on a camera's sensor, plus effective differences in DOF due to the change in formats. DOF itself doesn't actually change, but you tend to work closer to subject to fill the frame when using FF camera, assuming you are using the same focal length lens. Working in closer means shallower DOF, so being able to offset it with smaller aperture helps.

Years ago I stopped trying to calculate magnifications and such with macro... I just have a pretty good feel for what a tube will do on a given lens, from practice. Basically, with an 85mm you will want 12mm for mild increase in magnification, 20mm for moderate increase and 36mm for fairly strong increase. You'd likely have to use two tubes together to get all the way to 1:1, but most people don't shoot all that much at full 1:1 anyway.

It depends upon the lens' focal length. 12mm extension on a 20mm lens is huge amount of extension. Meanwhile 36mm on a 500mm lens is only a very modest increase in close focusing ability.

Basic rule of thumb is that you need extension equivalent to the focal length to get all the way to 1:1.... but this is also very much oversimplified. All lenses have some native close focusing abilitiy, so you'll find you actually need less extension. Considerably less with lenses that are pretty close focusing to begin with.

Canon has a lens chart on their website, and I think they show the effect of their own 12mm and 25mm extension tubes on magnification and/or close focusing distance with each lens. You might want to download that.

But, really, IMO the best way to learn to use the tubes is just to get busy using and experimenting with them.

Wow!
Thanks for that very informative post.