Photographing Insects: Some Theory
Knowledge of some theory on macro photography will help
I like to understand precisely what I am doing. I think it helps to know a little theory about macro photography when photographing insects. In this article I will explain some basic stuff about magnification factors, focus distance, aperture, and depth of field.
An extended version of this article has appeared in my free book My Journey into Insect Photography that you can download here.
Magnification factor
When you read about macro lenses they always talk about the maximum magnification factor. A macro lens should officially have a maximum magnification of at least 1x1, though many don’t. So what does this mean and how important is it?
Simply put, when the smallest area you can photograph in focus has a width of W mm, the magnification factor is 36/W. So when you can photograph an insect with a length of 36 mm and it precisely covers the image from left to right, the magnification factor is 1, or 1x1. When you have a magnification factor of 0.5x1 (or 1x2), the length that fits is 72 mm. When the factor is 2x1 (super macro), the length is 18 mm.
Why 36? Well, old analog cameras used film where each negative is 36 mm wide. So a magnification of 1 means that the size of the insect on the film is exactly the same as the size in real life. (When you print the image, the size of course becomes a lot larger.) For full-frame digital cameras, the sensor is also 36 mm wide, so the same applies there.
But what about APS-C cameras? These have a smaller sensor of some 24 mm wide. They have a so-called crop factor, which indicates the size ratio between the sensor and a full-frame sensor. This is often 1.5. For Canon cameras it is 1.6. If you have a lens with a maximum magnification factor of 1, the area that can fit in focus on the sensor can be 24 mm wide. So the actual magnification is the crop factor times the magnification factor. For my Canon R7 this becomes 1.6x1.
With the current pixel density of the sensors, you can easily crop further. My Canon R7 has 32 megapixels. If you want to print images in excellent quality at a size of 30x20 cm (12x8 inch) or display them on a 4K monitor, you need only 8 megapixels. So you can crop the image with a factor of 2. The combined magnification factor now becomes 3.2x1.
A normal, non-macro lens, sometimes stil has a decent maximal magnification. My Canon RF-S 18–150mm, has a magnification of 0.47x1 (at 50mm focal length). With the APS-C sensor this become 0.75 and with digital crop more than 1. So, if you allow for some cropping, you can still do very decent macro photography with it, as the images in the article show. And there is an additional advantage related to depth of field (see below).
Minimal focus distance
The maximal magnification is reached at the minimal focus distance, that is, how close you can get to your subjects. This distance depends on the magnification factor and on the focal length (indicated in mm on the lens). For example, if I set my 18–150mm lens to a focal length of 60mm, the maximal magnification is about 0.4 and the corresponding minimal focus distance is 21 cm. When I set the focal length to 150mm, the minimal focus distance becomes 45 cm. If I use my 100–400mm lens at 400mm, it becomes more than a meter.
Note that this distance is measured from the sensor, not from the front of the lens. My 18–150mm lens is about 9 cm long and together with the body, I have only 10 cm left between the subject and the front of the lens. So you have to get up real close. This can also hamper the amount of light falling on the subject. (Avoid using a lens hood.)
If you need to stay further away from your subject you need a lens with a (much) larger focal length. With high magnification factors, the difference though is not that large. For example, the Laowa 65mm macro lens that I might buy in the future, at a magnification of 1, has a minimal focus distance of 18.5 cm, while the alternative Laowa 100mm has a focus distance of 26.5 cm. That lens though is 2.5 cm longer (and much wider and heavier), so you win only 5.5 cm. And this 5.5 cm is not going to help to keep the insects put.
Depth of field
The depth of field (DoF) of a lens at a certain magnification factor (or focus distance) is the distance between the front and back of the area that is still in focus. The definition of depth of field is rather complex, based on the concept of circle of confusion (would be a good movie title).
The larger the magnification factor, the smaller the depth of field. The depth of field also strongly depends on the aperture (F-number). The narrower the aperture (higher number) the larger the depth of field.
As an example, again consider the Laowa 65mm. We look at the depth of field for different magnification factors and apertures:
- Magnification 0.5. F11 gives DoF 3 mm, F4 gives DoF 1 mm.
- Magnification 1. F11 gives DoF 1 mm, F4 gives DoF 0.35 mm.
- Magnification 2. F11 gives DoF 0.37 mm, F4 gives DoF 0.11 mm.
As you can see, these number are really small. Of course, the subjects are small as well, but still. This is the reason you have to use a narrow aperture for macro and focus very precisely.
Using an APS-C sensor does not affect the depth of field. The same is true when cropping the image. So using an APS-C camera and some digital crop, you get a much larger depth of field than when using a higher magnification lens.
Sometimes a small depth of field is actually nice. It guides the viewer to look at exactly the area you want her to focus on. Often this is the eyes of the insect, but that is up to you as a photographer. In that case, use a wider aperture. The aperture also has influence on the background. A wider aperture often gives a nicer, more blurry, background that makes the insect stand out more.
A technique to remedy the lack of depth of field is focus stacking. Here you take a number of images at slightly different focus distances and combine the parts that are in focus. Fortunately modern cameras have some built-in support for this and there are lots of software tools for it. But it requires the use of a tripod or a very steady hand. In future articles I will discuss focus stacking in detail.
Conclusion
I hope this gave you some insight in the theory behind macro photography. Important take-aways are:
- You don’t need a special macro lens to get started, in particular when you are using an APS-C camera with enough megapixels.
- You need to get close to your subjects unless you are using a lens with a large focal length, but that gives other problems.
- Depth of field for macro is very small, so precise (manual) focus is very important.
- Most of the time you need to use a large aperture of F11 or more. And because you also need a fast shutter speed, that means you need a lot of light. So you better use a flash, as we discussed before.
Next: Extension Tubes
Previous: Focus
Mark Overmars is a dedicated insect photographer that loves to share and regularly publishes about his work and about photography in general. You can visit his website at www.insectphotography.org. Download his free insect photography book at www.insectphotography.org/book.
