Thursday, 3 April 2008

How to get the Correct Exposure

Avoid Under and Over Exposure
Welcome back to Macro Photography for Beginners. I have decided to write a post about exposure. Getting the correct exposure for your photography is vital if you want to capture stunning images. The problem that occurs most often in photography is that the exposure will be too dark (under exposed) or too bright (over exposed). Unfortunately there is no magic cure for exposure problems, getting the exposure right is your primary job as a photographer. A digital camera can suggest the settings by measuring the amount of available light. This is not a guarantee for a good exposure because the automatic light metering system can be a little bit hit and miss. This becomes more evident when photographing reflective surfaces.

There are several ways to overcome your photographic exposure problems. Firstly I would recommend that you read my previous posts on aperture, depth of field and shutter speed. Exposure is much more difficult to get right when working in close up or macro photography. This is mainly due to the low levels of available light caused by the magnification and working distance. In general photography you can simply adjust the shutter speed, ISO or aperture to overcome exposure problems. When you are working with macro photography setting changes can have a negative effect on your composition. If you are attempting outdoor macro you may only get one attempt at the shot. Most mobile subjects do not hang around and pose for the camera!

So problems have to be eliminated before you get to the subject and set up the camera. I will tell you a great way to this later in this post. It does not matter how you intend to set the camera up. The easiest option is to use your camera in Aperture Priority mode (marked Av or A). In this mode the camera will adjust the shutter speed to match any aperture setting that you have entered. This eliminates studious guess work and means you only have to concentrate on setting the aperture with your depth of field in mind. A common problem with photography is that the actual end result will not resemble the image on your LCD screen. The camera uses a back-light to make the image display on the camera as a rough guide. Often the reality is that the image is much darker or much lighter. In bright sunlight it is very difficult to gauge the exposure of a shot using the LCD screen. The best way to check the exposure is to us the cameras histogram feature.

What is a histogram?
The histogram is a very useful pixel based graph of a captured image. The actual histogram displayed will vary according to what type of picture you have taken. In general a high amount of pixel activity to the left of the histogram will mean the image is under exposed. A high amount of pixel activity in the centre of the histogram means the image may be correctly exposed. A high amount of activity to the right of the histogram means that the image may be over exposed. The results depend on the image taken and this has to be considered when viewing the histogram. A properly exposed night scene will have a lot of pixels to the right. A picture of a white building will have a lot of pixels to the left but could be perfectly exposed. The histogram feature has its limitations but can be a very useful guide to how well your settings have worked. If you camera has a histogram feature, find it and figure out to use it and you will soon get the hang of getting better exposures for your photographs. Some of the new cameras on the market have much better (and larger) LCD screens than the older models. There are some ways to get around the LCD and bright sunlight problems. My advice is to forget about flip out screens and those angled optical extensions, and just learn to use the histogram. Remember that this applies to all digital photography and not just macro. Once you have established that there is some kind of problem with the exposure you will have to recompose the shot and take the picture again.

Figure 1. Example of Histogram Screen (Auto Exposure Bracketing +2 Compensation)

Auto Exposure Bracketing (AEB)
Auto Exposure Bracketing is a feature that allows the photographer to take the same photograph with multiple exposures. Most digital cameras that are capable of macro offer some type of exposure bracketing. This means that you can set an equal amount of negative and positive compensation. The camera produces 3 images of the same frame (or more depending on the exact settings of your particular camera). In this example I will set my camera to take a picture with 2 stops of compensation. This means that the camera will take the first image -2 stops, the second image at 0 stops and the third image +2 stops (see figure 1). This image had the best exposure out of the 3 images. Now before you start jumping up and down yelling “hallelujah” thinking you have solved all your exposure problems, Auto Exposure Bracketing does have a few drawbacks. The main one is that is does not work on my camera at all! I think this is a bug (or a glitch) with my camera. I have a friend called Jim who used a hacked version of firmware on his canon 300D and this is said to have caused his Auto Exposure Bracketing to stop working. Please keep this in mind if you want to follow Jim’s example and use the firmware hack on your camera. If you do use the hacked firmware then your camera warranty will no longer be valid. If you have a second hand Canon 300D (Rebel XT in the US) you should check that you have the correct firmware installed. To do this select the “Menu” button and navigate to the second tools and settings tab (orange). The firmware version will be displayed here (example 1.1.0). It is fairly old news about the firmware hack so I will leave it at that for now. I just wanted to make the point that if you have hacked the firmware your auto bracketing might not work, which brings me to my last point. Auto Bracketing Exposure does not work in any of the auto modes, only Manual. It may be different on some cameras but Auto Bracketing Exposure does not work with any type of flash photography. In macro photography Auto Bracketing Exposure is as much use as a pair of chocolate underpants. It is more useful to dial in your own compensation value using Aperture Priority or Manual mode and using your off-camera flash at the same time when/if required.

Do You Need to use Flash Photography?
I think we have established that Auto Exposure Bracketing (AEB) is not the answer to our ongoing exposure problems. If you are serious about getting professional looking macro photographs you need to invest in a decent flash system. I chose to use a Sigma EM-140 DG Macro Ring-light Flash for my own purposes - which are mostly botanical. This is a great piece of kit for any beginner in macro photography. Having said that, it is important to find the equipment that suits your needs rather than just buying the most expensive or aesthetic gear. Learn how to get the most out of the camera and equipment you already use. The ringflash enables good exposure in all situations and with faster shutter speeds with varying aperture values. This usually results in sharp correctly exposed images which is what all photographers crave!

It is important not to become too reliant on flash photography as you can get some very good photographs without using the flash. It is always better to capture an image that looks as natural as possible. To simulate natural light with a flash gun can be a difficult task. In many cases the flash will be too powerful and result in unwanted bright spots or catchlights. Catchlights (also known as catch lights, eye lights or Obies) are often used in portrait photography to show light reflection in the subjects eyes. It is a useful creative effect when purposely employed. In macro photography the aim is generally to capture and retain as much detail as possible. A small amount of catchlight in the eye is acceptable. Additional bright spots (often caused by a ringflash, twin flash and flash guns) are considered to be exposure defects under most circumstances. Try To avoid unwanted bright spots when photographing reflective surfaces by making the flash less powerful and/or altering the angle of the shot. To diffuse the flash you can cover it with anything that will make the lighting softer. The plastic from a supermarket milk bottle or a sheet of tissue or tracing paper are quite good material to use. I recommend taking shots of highly reflective objects using different settings and techniques. Diffuse the flash and see the effect it has on the image. This will give you confidence for the next time you discover a very shiny beetle or fly. There are several other options to consider such as light tents, light boxes and polarizing filters. I plan to investigate all these options (and more) in subsequent posts. Lighting is a huge topic in relation to macro photography and deserving of a full post of it's own. In the end it comes down to using just enough fill-in flash to pick out the fine detail of the subject without washing out the exposure. Try taking some macro (or close-up) photographs of reflective objects. Stuck for ideas look around your home for anything metallic or made from glass such cutlery, jewellery, bottles and mirrors.

Figure 2. Close-up of a vegetable stock cube.

Taking Macro Shots without using Flash
If you have not got around to buying a dedicated flash unit yet here are some methods for you to try. Place a tube the length of your camera lens over the pop-up or built in flash. Seal any gaps with an appropraite adhesive tape. This slight modification will cause a burst of light when the flash fires close to your subject. It is not a perfect solution by any means but it can be effective for some still-life subjects. Another option for still-life indoor macro photography is the use of continuous lighting. This can be achieved without the use of expensive studio equipment. If you want to avoid shadows you will have to use more than one light source or use a large amount of diffused light from a solitory source. Experiment with lighting effects and see what happens to your photographs. I often use this type of lighting to illuminate still life subjects such as coins or stamps during indoor photography sessions.

Try lighting your subject with an LED torch (or flash-light) or any type of lamp or light. Please take care when using home made continuous lighting equipment and/or diffusers. Make sure that any lighting that you use is wired correctly and is safe to use without causing a fire risk.

Macro Photography for Beginners
I hope you have found this article useful and informative, hopefully it will be help toimprove your own digital macro photography. My own achievement has been writing this entire article without adding any jokes about indecent exposure. Another challenge I am facing is going to be finding time to carry on writing articles for this web-blog. I like to write big useful articles that give readers something to take away and think about. A large amount of macro photography websites are written as an after thought as part of general photography website. I am striving to make this a worthwhile resource based on my own knowledge and experience as a photographer. It is my hope that this comes across in my macro photography articles. In my next post I am going to write about the importance of composition.

If you have any questions, comments, queries or suggestions please send me a quick email or write in the comments box.

Marvin Africa
Freelance Writer and Macro Photographer


Anonymous said...

Hi Marvin, I loved reading your advice on macro photography. I am having trouble getting the right exposure on my macro shots. I have recently purchases a MR 14x canon ring flash, and also have a 100mm macro f/2.8 USM lens and a Canon 40D camera.
I am using macro for intraoral dental photography as I am a dentist.
I am trying to shoot in the F16-F32 range to keep my DOF as high as possible so I can have as many teeth in focus as possible. however, in aperture priority mode, i select the right F setting, but the camera automatically picks my shutter speed and the shutter speed is too slow (almost 1second at times). the flash fires off, but the pics have movement artifact.. i tried this with iso 400 and iso 100. when i go to manual mode and pick shutter speed at 1/60th and F16 and shoot it, the flash fires off and gives me slightly underexposed but crisp pics. Next I tried to keep the 1/60th SS and F16 and went up on my ring flash exposure compensation to +2 and the shot is focused and well exposed. I am not sure why I have to go up on the exposure compensation of my flash to correct the aperture. should my ring flash on ETTL mode automatically fire the right intensity for correct exposure. I am confused. Please advice. Next I tried to go to F32 and same problem - underexposed pics. This time I achieved correct exposure by going up on my ring flash exposure compensation to +3.

Marvin Africa said...

Hello Ankur,
This is a tough one! Firstly let us assume that you have a correctly calibrated PC monitor to view your images on. Cheap CRT monitors make everything look under exposed so they are best avoided. The reason for my delayed reply is that I wanted to replicate your settings in my office. I was photographing an old USB hub and not someone's teeth but the principle is the same.

My advice is to move into full manual mode. This will give you full control of the camera and ring flash. Don't worry, it is not as scary as it sounds. Set the camera to manual mode (M) and set the shutter speed to 1/100 (it may be displayed as 100). Set the aperture to your own preference, I used F22. Switch on the ring-flash and set it to manual mode. In this mode you have control of the intensity of each flash (left and right). The intensity of the flash will be displayed as a ratio on the LCD screen. I'm using a Sigma EM140DG but I imagine that the Canon is very similar. Take a test shot using these settings and then experiment with the intensity of the flash. This is the range on the sigma ring flash 1/1,2/2,4/4,8/8,16/16,32/32,64/64 - 1/1 = highest and 64/64 = lowest. Keep the intensity equal on left and right, otherwise you may lose some detail to shadows. Manual mode is the quickest and easiest way to set-up and use a ring-flash.
Now you can adjust shutter speed and aperture for each individual shot. If you shoot in RAW format you have some additional options to make slight adjustments to exposure afterwards. This is really useful if the images are just slightly out either way - under or over exposed. The image can then be converted to a JPG and/or other file formats of your choice.

The use of a faster shutter speed and more flash will provide sharper images and allow for some slight movement in the subject.

I hope this helps, let me know how you get on.