Tuesday, 30 September 2008

Why most lens tests are flawed

Several lens test sites have sprouted in recent years. The test results published by these sites are taken seriously by many users, therefore, it is important to understand what value we can attach to these tests.

Firstly, the lenses are tested at a particular focal plane, i.e., the distance between the lens and the test object is fixed. As we all know, in the real world, a lens is focused anywhere between its closest focusing distance and infinity. To test a lens at a single focal plane and then to extrapolate from this an overall judgment of the lens performance is flawed in my opinion.

Secondly, certain lenses are designed for specific types of photography. Such as macro lenses for macro photography, short telephoto lenses for portrait photography, long telephoto for sports/wildlife etc. To test a lens outside of its usage context seems pretty pointless. One has to understand what purpose the lens is to serve, and devise tests accordingly.

I suggest that anyone interested in lens design should read the articles published under Nikkor Thousand and One Nights. Each article describes the goals that were aimed for during the design of a particular lens. This gives you an appreciation of why one-size-fits-all type of testing is so inadequate.

In my experience the real life performance of a lens cannot be judged by any of the tests published online. It is best to try out a lens yourself, perhaps rent it from a rental shop before buying it.

The Multi-CAM 1000 sensor

This was first posted in May 2008

There does not seem to be much information available on the D80's Multi-Cam 1000 sensor. I have had a few pictures that are out of focus, even though the focus point is where it should be. At first I blamed the Sigma 18-50mm lens, but now I am beginning to think, it is the AF sensor.

The first issue is that the focus points that are to the left and right of the central area can only detect horizontal lines. I tested this, and found that they simply don't detect vertical lines.

Secondly, the focus central focus area itself is slightly bigger than the bracket displayed on screen. If multiple objects appear inside the area covered, then the camera can end up focusing on the wrong thing.

An out of focus picture is a real disappointment as it is not always evident until you look at the picture at 100% magnification. By then it is too late to fix it. Hopefully as I learn more about how the D80 focuses, I will be able to get more pictures in perfect focus.

I am surprised that none of the D80 reviews really look at its auto-focusing abilities in depth.

Update: I asked Nikon Support about the layout of the AF sensors. The answer I got was this:

The centre AF area is a cross type area. The 2 AF areas above and below the centre area are horizontal AF sensors and all the rest are vertical AF sensors.

This means that the 2 sensors above and below the central sensor can only detect vertical lines, and the rest of the sensors, apart from the central one, can only detect horizontal lines. I have to see if I use this knowledge to get perfect focus each time.

Sigma 18-50mm F2.8 EX DC HSM Macro review

This was first posted in May 2008

I bought this lens a few days ago for my new D80. Here are some observations after testing the lens both indoors and outdoors.

It is soft at F2.8, even at the centre, especially at the 50mm setting. At F2.8, performance is better at the wider focal lengths.

Generally, performance seems to improve quite a bit by F4, gets noticeably better by F5.6 and improves even more by F8. I have not printed any of the photographs yet - so do not know what the acceptable maximum print size would be at each aperture/focal length.

For majority of general photography, I would probably stick to apertures F4 and slower, and only use F2.8 if the alternative would be to not take a picture at all.

Build quality seems good - although there are two tiny specs right at the border of the front lens - so small that they are insignificant, but I was not expecting a brand new lens to have this. It was shipped to me direct from Sigma UK - I was surprised at how poorly it had been packed for shipment.

Focusing is silent. Focusing speed is okay for my purposes. There is no manual override in AF mode - despite the fact that this lens uses an HSM.

A few samples taken at f/2.8 and f/8 are posted here. These show the relative performance wide open and stopped down. You will notice that at 50mm, there is a marked degradation in performance at f/2.8.


dibyendumajumdar's Sigma 18-50mm Test photosetdibyendumajumdar's Sigma 18-50mm Test photoset

It was agonizing to choose between this lens, the Tamron 17-50mm, and the Nikkor 16-85mm. I ultimately decided on the Sigma on the basis of better build quality than Tamron, and faster speed than 16-85mm Nikkor. Sadly, the fastest aperture is not as usable as I would have liked.

I think that the tests and reviews available are often misleading (especially Popular Photography review of this lens) - as they do not reflect the real performance of this lens at F2.8. The review that I would say is reasonably accurate is:


Things that I wish the Nikon D80 did differently

This was first posted in April 2008.

Here's couple of things that I wish were different with Nikon D80:
  1. It doesn't meter with non-P, and non-AF lenses. I did not realize this when I bought it. So this means that if I use my Zeiss 50mm f.1.4 lens, I have to guess what the exposure ought to be. This is pretty bad as I am no good at getting the exposure right without a light meter.
  2. There is no ability to store a favourite configuration, like I had with my F70. This was a really useful feature - as it allowed me to quickly go back to a preferred setup. Ken Rockwell suggests a workaround in his tips on using the D80.

D80: Initial Impressions

This was first posted in April 2008:

The viewfinder is small. I had read about this, but it still came as a shock to me. Before deciding on the D80, I was looking at buying D60. I am glad now that I did not, because with the D60, I would have had to manually focus my existing lenses, and given the size of the viewfinder, this would be very difficult.

Another curious thing I noticed that when I had attached my lens, initially the viewfinder appeared very dark. It seemed as if the lens was stopped down. Once I had put the battery in, and switched the camera on, the viewfinder became bright.

Update: This is documented in the manual. Apparently, the viewfinder goes dark if the battery is not inserted or if the battery charge is low. I wonder if this is because the camera needs power to keep the lens at is maximum aperture.

Twenty years with Nikon

The first Nikon I had was when I convinced my father into buying me a second-hand Nikkormat. I had to have the camera replaced because it was faulty. The replacement was a Canon A-1.

I dreamed of owning a Nikon. By a stroke of luck, someone who worked with my Dad had an old Nikon F Photomic and a 55mm f/1.2 Nikkor which was lying unused. He offered to sell it to us for what seemed to me a very good price.

I used the Nikon F for many years until it went faulty and the cost of repairing it was higher than buying a new camera. For short while I owned the F50, replaced it with an F70, and now I own an F100 as well as a D80, which I acquired early this year.

I waited to buy my first DSLR because I felt that the price/performance ratio just wasn't right. An SLR lasted you for years, and all you did was buy lenses. DSLRs become obsolete in two years, and there is enormous psychological pressure to upgrade, just like your laptop, or your PC.