Over the years I've had many philosophies on how to approach my everyday photography. With every new venture out in the field, or new subject identified, that philosophy changes. Usually just a little bit at a time. Some time ago, I started to drift into getting the sharpest image possible. That sounds bad, like I started with getting the most out of focus image I could and I would be happy with it. Like all things photography, we're talking degrees. If we understand how and why aperture is a factor in our environment - those degrees will start to fall in our favor. And the path to a sharper image started.
First, what is aperture? Aperture, along with shutter speed and ISO are the three main factors that balance our images. Aperture is what controls the amount of light in the process. In my opinion, in most circumstances, control of aperture is the most important factor in the balance. You would think that in shooting my favorite subjects shutter speed would be my initial consideration, but it is not. And I'll cover how I got to that statement.
Technically, aperture is the term associated with the opening of the lens diaphragm. Sorry if this takes you back to high school but the opening measurement is a geometry thing. The measurement of the opening (area) is pi = radius squared. Same for all lenses. That number becomes part of the focal ratio. It is the number we control in this ratio. The second number in the ratio is related to the focal plane and some higher level physics that honestly confuses me. So, since I can't do much about it - I ignore it. But the focal ratio morphs into the term we associate to aperture setting, f-number. The f-number is the reciprocal of the relative aperture (the aperture diameter divided by focal length (1)). And in photography slang, f-stop.
And what a photographer needs to know in the field as a camera operator is the lower the number the camera displays the larger the diaphragm opening.
And what a camera operator needs to know in the field as a photographer is how the diaphragm opening affects the photographic process.
And here is where we get into degrees. In some of these areas, we're going to get into trade-offs. If you gain here, you might lose something there.
What does having a lower f-stop value do for you? Easy answer, lets in more light. I'm a big fan of more light. More light, faster shutter speed and lower ISO. It's all good - except. Yea, that pesky Depth of Field thingie. I could probably do an entire post on DOF but for now at the most simple definition is DOF is what is sharp in a photo. Small f-stop number, large diaphragm opening, short DOF. Physics, nothing artsy about it.
The characteristics of the lens are very important. Your purchase of a lens is based on the max aperture (largest diaphragm opening). The max aperture is part of the lens name. I could devote another post on how to read how lens' name.
But DOF is the easy answer when asking what does the aperture setting control. However there are other considerations when answering that question as well. And this is where we get into degrees. Most of the degrees we're talking about here can't be seen, unless you are looking for them.
A camera lens is designed to let light through. But the last object on light's way through the lens is the restricting diaphragm which is controlled by the f-stop set by the photographer. And that's where the light gremlins strike. A small (maybe) list of other aperture setting characteristics.
Auto-Focus: Most newer cameras autofocus with the aperture wide open. Right before the shutter is activated, the diaphragm moves as selected. If your camera's auto focus does not do this, as the aperture is closed down autofocus may suffer. When looking at reviews for the 200-500, I considered Tamron and Sigma's lenses in the same category. Many of the reviewers would go into AF and the best situations for each lens. Again, degrees.
Diffraction: Diffraction is a physics concept that happens when the path light takes to the sensor is altered. Light is 'managed' through the lens by various internal lenses. The path is is even more complicated in a zoom lens configuration. But light is managed until it hits the diaphragm. You can think about light going through a lens like water going through a hose. With no attachment at the end of a hose, water flows freely in a smooth stream. Put any attachment at the end of the hose, and the water flow is altered disrupting the smooth stream. From this analogy you can tell that the more restrictive the opening at the end of the hose, the more the water flow is altered. In a previous discussion we had about noise on the sensor, this also disturbs the light on the sensor. Remember, we're talking about degrees of disruption here. A photo shot at f/22 is still useable. There are situations where f/22 might be a necessity such as everything needs to be in focus. But if you're using f/22 for something else, there are probably alternatives. Diffraction isn’t a huge problem, but it exists. For me, I try not to drift above f/8. I try to be conscious of what my DOF needs to be and change accordingly strictly for diffraction considerations. But if I need to make the DOF tradeoff, I will. And the photo will still be fine. Just when pixel peeped, not as fine.
Added diffraction note: There is a multiplied effect on a sensor smaller than full frame. On my D500, the crop factor is 1.5. With a full frame lens, 50 mm is 75 mm. With a DX lens on the D500, f/11 would be roughly equivalent to f/17 in a full frame environment. Not sure what the effect is when I use a FX lens on a DX sensor. As most diffraction occurs around the edges of the frame, the smaller sensor takes it's light from the center of what light is let through the lens. Edges are not captured because it falls outside the sensor area. The situation is probably close to a wash. Degrees again.
Lens quality/Aberrations: Aberrations are fun to talk about. For me, aberrations are issues with photos that I didn't cause by poor technique or over thinking the situation. Aberrations for this discussion can be characterized as optical issues. Stuff you can't fix. Hubble telescope, original version, stuff. As mentioned in the previous paragraph, aberrations are more common in the corners of the frame. Some common aberration types are spherical aberration, field curvature, coma, distortion, astigmatism, color fringing. So how does the aperture setting inflict this much damage? Simple, the diaphragm blocks light by closing the light path area from the outside. Take light away from the outside edges. The edges of the frame is where the disruption happens.
Dust specs: Dust specs tend to show up more with smaller aperture openings. For dust specs on your lens, mostly DOF. A dust spec that close to the sensor is a bit larger that a dust spec at 10 feet. And dust specs will show up on a sensor at f/16 before they show up at f/4.
So what does this mean? Take a breath. It's all good. If you need f/16 - use it. But know what f/16 will cost you. The photo will still be fine. But if you are taking a landscape shot where you need full focus, you can use f/22 - or learn photo stacking. If you want to block light, try a neutral density filter. I try to keep my apertures in the f/3 to f/5 area. And as mentioned before, I'm willing to trade a higher ISO for that setting. I set my aperture first - which would make it the most important setting.
See how I tied that up? It's all good.
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