aperture

GX1 Depth of Field

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Shot on a Panasonic GX1 with Olympus 9-18mm lens set to 10mm. I used f/9.0 for this image and whilst its sharp it would have been better at f/7.1. I didn’t have my depth of field scale with me and couldn’t take any chances. Click the image to see a larger version.

I am growing tired of seeing the same advice trotted out time and time again about Landscape Photography. The advice that has me so wound up is that “if you want to get a full depth of field you need to use a very small aperture”. This advice has almost been carved in tablets of stone for all to see. Does it work; yes it does BUT IT ISN’T THE BEST ADVICE.

To me, this advice is almost the lazy way to photograph landscapes. The advice you should be getting is to shoot your landscape with the best aperture (the one that gives you the highest quality image) whilst still giving you the required depth of field. To understand why and why this is so important for Micro 4/3 and compact camera photographers you need to understand a few basics about lenses and depth of field.

Firstly most consumer lenses are designed to achieve their sharpest images and resolve the most detail when the aperture is closed down about 2 stops from wide open. For many lenses this means you will get the best results at around f/8 to f/12. Either side of this range you will find the image just isn’t as sharp and if you stop down the lens to around f/22 you might find all sorts of distortions can kick in. Buy the best lenses you can and you might see some improvement on this.

Now the thing with Micro 4/3 cameras is they seem to hit their best performance when stopped down just a little bit and the best way to understand this is by testing your own lenses. I haven’t done extensive testing but with my 14-45mm lens, I tend to shoot landscapes at between f/5.6 and f/7.1 for the 14mm end of the zoom range. I have found that if I stop down to f/8 or beyond then it’s not quite as sharp.

So you know the aperture range where your lens performs best, how do you ensure there is sufficient depth of field? The answer is by selecting the correct point of focus first and then selecting the correct aperture in this range. This is probably best explained with an example.

Suppose you find a nice landscape scene and there is a lovely rock that you want to place in the foreground. You compose the shot with your camera mounted on your tripod at just below eye height and the rock is about 5 feet from where you are stood. The distance from your camera to the rock is probably going to be about 7 feet. If I consult my depth of field chart (it’s actually an app on my phone) I find that focussing on the rock with my lens set to 14mm (on the GX1) the widest aperture that allows me to achieve infinite depth of field is f/6.3 – yes you read that right. The nearest point to me that is in focus is actually only 3.4 feet away.

Taking a more extreme example lets lower our tripod and move in close to the rock so that it looms large in our viewfinder. Unfortunately the 14mm lens isn’t wide enough so we move the Olympus 9-18mm set to 10mm. This is the equivalent of a 20mm lens on a full frame camera. Having moved in close we find our point of focus on the rock is just 4 feet from the camera. Looking at my depth of field app I find that I can achieve infinite depth of field at just f/5.6 and the closest point in focus is just 1.9 feet from the camera. At this aperture my lens is going to be performing at its sharpest and resolve the maximum amount of detail. The images produced will be substantially better than if I had used a small aperture such as f/16. Just for fun I thought I would check the aperture I needed with my 5D and its f/11.

Do you think you think you have been selecting the best aperture for your work?

Select the Right Aperture – Part 3

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The final post in the series…

Compact mirror less cameras such as Micro 4/3 are slightly different from this. Most seem to be good performers from wide open, hit their best performance when stopped down by one stop and then gradually tail off as diffraction kicks in. The cameras do however have a huge advantage in terms of their smaller sensor size increasing the depth of field. A typical Micro 4/3 camera has a sensor with a 2x magnification. This also means my depth of field is also effectively doubled. I know that at f/7.1 on a 14mm lens (28mm equivalent) I can achieve a full depth of field. My LX5 has an even smaller sensor so by the time I have stopped down to f/3.5 at 24mm equivalent focal length I can achieve sharp focus from 1m to infinity.

So how do you use this advantage?

Firstly understand how your lenses perform at each of the apertures. When are they at their sharpest and when do they suffer from problems such as diffraction.  This gives you your ideal range which you should try to keep within.

Now select the focal length of the lens you will use. This has a big impact on depth of field with longer lenses having less depth of field than wide angles. I suggest selecting the focal length first as I see this as a more important consideration in composition than depth of field.

Once you have composed your image consider how much depth of field you need to achieve. A number of important factors come into play here:

  1. The focal length of the lens as wider lenses give a greater depth of field than telephoto lenses at the same aperture
  2. How far you are from the closest point you want in focus. The nearer this is to the camera the less the depth of field.
  3. The size of your sensor as small sensors give a greater depth of field at the same aperture than larger sensors
  4. Where your point of focus is. The depth of field at a given aperture extends roughly 1/3 in front of the point of focus and 2/3 beyond.

The reason I was able to shoot my New York Skyline image at f/2.8 is that my point of focus was at infinity and I was shooting at the wide angle end of my lens. These points alone were enough to give me the depth of field required. Once you have mastered the points above you suddenly realise the common wisdom of stopping your lens down to its smallest aperture often isn’t correct and won’t give you the optimal image.

Select the Right Aperture – Part 2

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Image

Copyright: Robin Whalley, 2011

Continued from previous blog…

So, looking at the use of aperture to control depth of field is actually a very effective tool for doing this and depth of field is a key creative decision you need to make when capturing your images. Let’s say you want to create a portrait but the background is distracting. The solution is to open the aperture wide to throw the background out of focus and isolate the subject. If you are shooting landscapes and want to achieve the maximum depth of field so that everything from the foreground to the distant background is sharp, you would select a small aperture. Sure there are other factors at work here which can emphasise the impact of the aperture but they do just that, emphasise the effect. The priority should still be to select the right aperture for the image.

Now let’s consider some of the problems that come with the aperture control and lenses. The first occurs when we stop the lens down to its smallest aperture. This often results in diffraction and potentially distortion. I once had a wide angle Pentax lens that when stopped down to its smallest aperture would bend and distort images in the corner. Diffraction is a little different and will result in soft and slightly out of focus images. Again I remember when I was first starting in photography and I had a cheap wide angle Tokina lens that suffered badly from diffraction. Not understanding this I would stop down the lens to f/22 and the results were always poor.

At the opposite end we can open up our aperture as wide as possible. Most lenses however don’t go very wide with a maximum aperture in the range of f/3.5 to f/5.6 (particularly true of zooms). At their widest aperture most lenses are also soft, especially in the corners. Many also suffer from uneven lighting and tend to vignette due to light fall off in the corners – not always something you want. Higher quality lenses from top manufacturers such as Zeiss and Leica tend to be the exceptions and will still perform well when wide open or stopped down. Unfortunately they are also expensive.

This then leaves most of us with an area of optimum lens performance which is usually 1-2 stops above wide open for usually 1-2 stops. With a typically zoom lens on an SLR this is probably in the range of f/8.0 to f/12.0. Does that sound familiar?

To be continued.

Select the Right Aperture – Part 1

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Image

Last Thursday night I was presenting at Bolton Camera Club. It was a good evening with lots of discussion, especially when I pulled out the sample prints I had taken. A lot of the members looking at the prints made from my LX5 compact Camera were surprised by the not only the quality and detail displayed but the “relatively wide” aperture I had selected. The image above was shot at f/8.0 yet it’s sharp from front to back. My New York Skyline shown in an earlier post was shot at f/2.8 and everything is in focus, even viewed close up printed at A3+.  The member of Bolton Camera Club are not however alone in this surprise as it’s something I experience often when talking to club members from all over.

Selecting the right aperture for a scene is something we tend to do automatically once we have been in photography for a while. Unfortunately the aperture selection in lightweight cameras might not be quite as you expect it and you could end up making choices that have a negative impact on your photographs. If you stop and think, there are only a couple of reasons why you might want to control your aperture:

  1. To control your depth of field
  2. To control your shutter speed (increase or decrease)

Now I’m going to be controversial and say the only reason you should be changing your aperture is the first one. The second option, to control your shutter speed is in fact a trade off from controlling your depth of field. If you disagree good (you have your own opinion) but stay with me and understand my argument. It might just change your perspective on photography.

So let’s deal with the “misconception” of using aperture to control shutter speed and why I argue against this. My reason is that it’s actually a relatively ineffective when used in this way. If you want to change the shutter speed for creative reasons (which should be your first priority) it will be because your current speed if either too slow to freeze action or it’s too fast to introduce creative blur and motion.

Let’s say I want to freeze motion so I open up my aperture as wide as I can. This will increase the shutter speed but at the expense of reducing the depth of field in the image, something I might not want.  Often such a reduction in depth of field will affect the aesthetics of the image and will change it completely. A better option is to increase the ISO setting as this is more effective. It will introduce noise but you can always address noise if it becomes too great in other ways. Another solution which is often overlooked is to use flash to help freeze motion. In the end you might need to adjust the aperture as well as these solutions but this needs to be a deliberate decision and you need to be aware of the impact on your image as well as the drawbacks.

Now if you want to slow the shutter speed you might try to stop down the lens to as small an aperture as you can. This will however introduce diffraction as we will cover later as well as affecting the aesthetics of the image. Rather than this approach try reducing the ISO you are using. Again, this might not have sufficient effect or you might already be shooting at the slowest ISO your camera can support. The best option however is to use a Neutral Density (ND) filter placed over the lens. These come in various strengths up to 10 stops and are incredibly useful. An alternative option might be to use a polarising filter which usually takes out 3 stops of light.

To be continued.