Well perhaps I'll get the ball rolling then with some photographic theory to see where this
thread can take us.
First, let's talk about aperture and depth of
field.
In your high
quality camera lens (typically NOT a smart phone camera), there are helical blades (called aperture blades) and lens elements.
The lens elements are arranged to collect a lot of light from the image
area, and to then focus that onto what we used to call a film plane. In the digital
area, we call this the sensor surface. The lenses, generally, are sharpest in the middle of the lens. As you get out towards the edge of the lens elements themselves, all lens element stacks start to see some level of distortion where the light entering the lens is refracted through the elements onto your camera sensor. Usually the more expensive and high
quality the lens, the more went into the design and manufacture of the
glass elements to reduce this effect. Cheaper digital cameras try to compensate for this with
software correction that is never quite as
good as capturing the image accurately to begin with. If you shoot in RAW mode and compare it to internally processed jpeg images, you would see this difference manifesting usually around the edges of your photograph.
The aperture blades are designed to close to a given
diameter when you take your photograph. Each lens expresses these values as "f-stops". The lower the number, the greater the
diameter of the opening the light can pass through when you take a photo. The higher the number, the smaller the
diameter. I'll explain more, but in simple terms - the higher the f-stop number, the longer the exposure, but more of the image will be in focus.
When you look through your viewfinder, these blades in your lens are wide open in default settings, letting the maximum amount of light through to reach the sensor. This high volume of light fully exposes your image quickly, allowing for faster shots that are fully exposed. Sports photographers and
portrait photographers value "fast" lenses that have very low f-stop settings numbers for this reason. This
comes at the expense, however, of depth of
field. Depth of
field is the distance in front and behind your exact point of focus - where your image is sharpest - where the rest of the image is acceptably in focus. When photographing, for example, people or scenery, this is usually not a problem. The focal plane is the distance between your camera lens and the perfect point of focus in an image. The longer to focal plane, the longer the depth of
field will be.
In other words, if you take a given lens and photograph someone's
face 2 meters away from you, and you focus on that person's eye - on basically every lens, their entire
face will be in focus - from the tip of the nose to their ears - no matter what settings you use on your camera, assuming your subject is focused on correctly. If you have your f-stop at the lowest setting, say anything under f2.0 on a full frame camera (or f4 on a micro four thirds camera), you will notice this person's
face is
still in
complete focus, but the entire background is blurred in a pleasantly artistic way. Photographers call this "bokeh" and people spend a lot on lenses to get this idyllic effect in some shots. If you change nothing else, but take the same shot at f22, you will notice two things. First, the camera shutter speed will be far lower because the camera has to let light in longer to get a proper exposure when there is only a tiny opening in the lens to let light through, and second, you will notice that the entire background and foreground of the photo (more or less) will be in
sharp focus.
So what does all this mean for coin photography?
Well it's not often that a coin photographer wants any
part of the coin to be out of focus. You are basically chasing an archival-quality reproduction of your coin, giving you a realistic interpretation of what the coin looks like in-hand. So you might ask, does that mean I should just shoot everything at f22? Much depends on the lens you are using, you lighting setup, how you mount your camera, etc. - but generally yes, you will usually shoot all coins at a high f-stop like f22. There are some exceptions, like if you have a focus-stacking camera - but most of you likely don't have one of these. The technology is new and
still quite expensive.
Some more things to consider - almost no camera will shoot at f22 if you leave the camera in automatic or program mode. Even in macro mode, it may not do this. My advise to all coin photographers - shoot in aperture priority or manual modes only. This gives you maximum control of depth of
field, and therefore, how much of your coin will be in focus.
This is sound advice for a typical flat coin, but what about a
scyphate coin?
Let me start with one of the reasons why scyphates are tougher than a regular coin. It has to do with the focal plane I talked about above. As you reduce the focal plane, your depth of
field gets much narrower. When we get to macrophotography, the depth of
field is now
reduced to a distances measured in millimeters. This means that if the height of your coin when laying flat is more than 2 or 3 mm, some macro lenses have too short a focal length to take a photo of a
scyphate coin where the whole
flan is in focus. In other words, now you need to worry about which macro lens to use to take pictures of these coins - not just any old macro lens will do. Your common 60mm or 70mm macro lens (full frame) may not give you the results you expect here.
I'm now going to switch to full frame equivalent numbers for focal lengths for this
part of the explanation. It's important to note that each camera system's focal length number on their lenses (for example, a zoom lens being marked 70-210mm f4-5.6) is only comparable to other camera lenses that are used on cameras with the same sensor size. Full Frame measurements are equivalent to the old 35mm camera
standard, so many people
still relate to these numbers most. If you shoot with micro four thirds, for example, you would divide these numbers by 2, and the equivalent lens to a full frame 70-210 f4-5.6 would be a 35-105mm f8-11.2. This gets trickier
still because many micro four thirds manufacturers list focal lengths in actual values (i.e. 1/2 the full frame equivalent), but list absolute f-stop values, which tend to make the lens seem "faster" than it really is. In the above example, if Olympus or Panasonic sold a 70-210 f4-5.6 full frame equivalent, they would likely market it as a 35-105mm f4-5.6. It can be misleading if you are not careful, but for coin photography it doesn't really matter - you will only use the slower end of your lens' range anyhow.
The human eye sees about what a 50mm lens sees. Anything more than that is a zoom lens, and anything less than that is a wide angle lens. Macro lenses use the same conventions, but are designed to be in focus when the subject is much closer to the front element of the lens. Now because we want
scyphate coins to be in focus, we want a macro lens, but one that puts the coin farther away from the lens than we would minimally accept for a flat coin - basically a "macro zoom". The farther away the coin is is, we have a longer focal plane distance with a correspondingly longer depth of
field - meaning a taller coin can
still be fully in focus.
You would have to experiment with your setup here, but I find the minimum acceptable focal length on a lens for a typical
scyphate is around 100mm full frame equivalent, shooting at f22. After that, the most important variables tend to be about keeping the camera steady and the lighting setup you use. If you use an even longer lens (more than 100mm), then there are some lighting advantages - assuming you either already have, or are OK with
buying, another expensive macro lens!
Here is a photo of a
scyphate Histamenon Nomisma taken with a (full frame equivalent) 100mm macro lens set at f44. It's actually a micro-four thirds 50mm lens set at f22, which is a 100/f44 equivalent in full frame. The distance from the coin to the lens element is 7 inches, or about 18mm. The coin is about 4mm in height when laying flat. I can improve the lighting of the
reverse around the
flan edge, but you will notice the entire coin is in focus for its full depth.
https://www.forumancientcoins.com/gallery/displayimage.php?pid=173453Happy to hear people's thoughts so far - soon we'll get into the magical world of teleconverters, extension tubes, flash setups, etc.
