In part 1 of this tutorial, I explained magnification in macro photography and the things you need to reverse an 18-55mm kit lens for high magnification macro photography. In this article, I explain the following topics:
How to measure the magnification of a reversed lens?
The usual question is, how do you measure the magnification once the lens is reversed. I have searched a lot about this. What I could find was that there is no absolute way to measure the magnification in macro photography. That is, there is no equation in which you could just put your parameters and get the magnification. All you could do is to use a ruler, take photographs at different focal lengths and then find out the magnification. However, I could be wrong about this. So if you have a proper method to measure the magnification of a reversed lens, then please share it with me.
F-number, magnification, and depth of field
The f-number, magnification, and depth of field are related. Understanding this will help you make a good image. However, this is most confusing part in macro photography and it is magnified in reverse lens technique and even more in high magnification macro photography. You will get confused about how to set the aperture or what aperture setting you should use at different magnifications to get sharp images. The usual mistake people make is ignoring the aperture settings when they use the reverse lens technique. And the result is an unfocused, blurred image. So you should know how to choose the aperture to get a proper depth of field at different magnifications. I can talk about this using some physics, mathematics and everything but I know it is boring and I don’t want to make you all dizzy and hate shutterstoppers forever. After all, getting good results doesn’t demand knowing all those stuffs. So, I am going to explain it as simple as I could do, even though there are some technical aspects which should not be overlooked. I hope you don’t curse me.
How depth of field is related to magnification and f-number?
Depth of field is simply the area of the image in focus. So, a shallow depth of field means very small area in focus and the rest of the image is blurred. The factors that determine the depth of field are aperture, the magnification of the subject, the distance between the subject and the sensor of your camera. Thus, by changing these, you can change the depth of field. But the story doesn’t end here.
High magnification macro photography needs a lot of light. This is because the effective f-number changes at high magnification as you need to use small f-number to get an acceptable depth of field. As I mentioned before, as the magnification increases, the depth of field decreases. At high magnification like 4X, the depth of field will be less than 0.2mm (that is the area in focus). So, to increase this depth of field, you need to use small f-number since narrow apertures increase the depth of field. But, even at f16, the depth of field is not good enough to get the entire subject in focus. In such cases, there are two things you can do: either focus an interesting part of the subject or use focus stacking method. However, it is not advisable to stop down beyond f16 since it causes diffraction which makes the image soft.
Now you might be wondering why at small f-numbers you don’t get very large depth of field even though you shoot landscape photographs at f16 and get the complete image in focus. This is because the f-number you set on your camera is not really the aperture setting at closer focusing distances (or when you shoot at high magnification). I guess you might be pulling out your hair now. Let me explain. The F-number you set in your camera is for infinity focusing distance. When the focusing distance decreases, the magnification increases. And when you focus at close distances, the f-number changes. Thus, the effective f-number is related to the magnification and f-number you set in your camera. You can see this by yourself. Set the f-number to f14 and change the magnification from 1X to 5X. You could see that the viewfinder gets darker when the magnification changes from 1X to 5X and vice versa.
Effective F-number =f-number x (1+magnification)
This is not a useful equation to calculate the f-number since there are several other parameters involved. This equation is just to give you an idea that f-number changes with magnification. Nikon users know this aspect more than Sony and Canon users since Nikon work on effective f-numbers. That is, Nikon changes the aperture as you focus closer while Sony and Canon does not. This is the case when the lens is in normal mount. In reverse mount, all brands work the same way.
Thus, high magnification results in shallow depth of field, so you need use narrow apertures (small f-numbers) to get an acceptable depth of field.
Changing the magnification while using a reversed 18-55mm kit lens
When the focusing distance decreases, magnification increases. Usually, lenses have fixed short focusing distance which is why you cannot take macro photographs using every lens. Dedicated macro lenses have mechanism to focus at close distances. In part 1 of this tutorial, I have shown you how the magnification changes with the focusing distance. The focusing distance is changed by:
- Changing the focal length of the lens.
- By an internal focusing mechanism. This technique is used in dedicated macro lenses like 90mm, 100mm, etc. When you focus closer using these lenses, the length of the lens doesn’t change.
So, at different focal lengths, you get different magnification. And the focusing distance changes, too. Thus, when the lens is in reverse mount, the focusing distance gets shorter when you change the focal length from 55mm to 18mm since the magnification increases.
Thus, the advantage of using a reversed 18-55mm lens is that you can change the magnification (1:1-5:1) by changing the focal length. Apart from getting different magnifications, another advantage is that you can find the subject to frame well. I will explain this in the next article.
All of this information might overwhelm you. So I thought I should give you the gist of this article. This article should give you:
- A fundamental idea about depth of field, f-number, and magnification in high magnification macro photography.
- Enough information to set the camera settings (aperture settings at various magnifications).
- An idea about how to change the magnification when the lens is in reverse mount.
I assume that, with a reversed 18-55mm kit lens, you need to take photographs at different magnifications and they should be sharp and aesthetically good. The information that is given in this article will not really help you to get the desired result. There are some issues you face even after setting the right aperture for the magnification and everything. You would understand this if you experiment with your setup. Some of you might get it at first, but mostly the results would be less satisfying as you would mostly get improperly exposed, blurred, inappropriately framed images. So, in the next article, I will give you some tips to make better images. But, in the meantime, experiment with the setup and let me know your results.