Photography tips: the sweet spot

Sweet spot, you may have heard that word before. You may even have been checking some of your pictures for it. You didn't findt it, did you? Sweet spot is not in the picture. It relates to the optimal aperture of a lens.

Optimal?

The term 'optimal' is a bit misleading here, because the optimal aperture depends on the type of picture you are trying to make. If you want a shallow depth-of-field, your aperture has to be larger than in the case you want a deep field. In low light situations, a large aperture (small f-number) is often the only option. But apart from these considerations, you may be looking for the maximum resolution (i.e. sharpness) of your lens. The aperture where the overall resolution reaches its maximum value is called the sweet spot. It is nice to know the sweet spot of your lens by heart, so that you can use it if you end up in a situation where you can. Shooting landscape photos in broad daylight would be such a situation.

Finding the sweet spot

But how do you find the sweet spot? It is not in the manual of the lens. It is actually quite simple. Find a review of your lens that contains a laboratory resolution test at different apertures. Photozone has them for all the lenses they have reviewed. Then check the resolution graph (or table) for the highest value and find the aperture that produces it. You have now found the sweet spot. Note that some lenses have different sweet spots for centre and border, and zoom lenses may have different sweet spots for different focal lengths. Then it takes some common decide to decide.

An example

Here's a quick example from one of my favorite lenses, the Canon EF 35/2. The figure (click on it to see it in it's context) shows that center sharpness is maximum at f/5.6, and border sharpness at f/8. That is not a clear verdict as to what the sweet spot is, but since border sharpness is below centre sharpness, I would say overall sharpness is best at f/8.

Photography tips: Lens and camera reviews

When you are going to buy a camera or a lens, and once you more or less figured out what type your after, the search for the best one begins. ‘Best’ obviously depends on your personal preferences as well, but you will need information. Review pages can give you this information, but there are so many different review pages. Some of them seem to be purely commercial, some are very technical and others consist of user reviews. How to find your way in this ocean of information? Let me tell you how I gather my information. I am not saying it is the best way per se, but it might be a good starting point to develop your own.

Once you know what you want, check three review sites. FredMiranda (FM) has a large number of user reviews. User reviews have the obvious advantage that they tell you how the lens behaves in day-to-day use. They have the disadvantage that a user may judge the lens wrong. The advantage of FM is that the number of reviews is quite large, so that a single misjudgment is not likely to have a huge impact. The overall score for any lens will give you a fairly good first indication, but you should read a couple of those reviews as well. Especially focus on reading the cons of the lenses and check whether they matter to you. If a lens is voted down for its noisy AF motor but is optically sound, you may not care as much as in the case of a lens that is voted down for lack of contrast.

A second site to check is Photozone.de. They have extensive technical reviews. You may either be interested in reading the entire thing, or just the verdict, that’s up to you. Note that Photozone lists all lenses by mount. That is no problem if you are a Canon owner looking for a Canon lens, but when you are looking for a third party lens (e.g. Sigma, Tamron, Tokina etc), you may miss out on the lens you were looking for. If you are for instance looking for the Tokina 10-17 for your Canon, go check the Nikon section. Photozone tested this lens on a Nikon body, so they listed it under Nikon. If it performs well on a Nikon body, it will probably perform well on a Canon body too.

DPReview mainly tests cameras, but they do have some lens reviews as well. For cameras, you can do a feature search as well as a side-by side comparison. They also keep track of lens and camera releases, which is also nice to know.

Apart from reviews, you may also want to check some pictures taken with the specific cameras. There are many sites to do this. I would like to mention the two sites that I check out for this purpose, but others are good as well. DPChallenge has a section called equipment, where you can look for the camera or lens you are looking for. On Flickr, go to the ‘groups’ section and type the brand and focal length (brand and type for cameras) in the search box.

If you still have questions, turn to a photography related forum. Members of those froum are often very willing to help and share their knowledge and experiences. All the sites listed above have such forums, and google is your friend in finding other forums as well, or forums in your own language.