A note on lens statistics

Following a couple of previous comments I feel I need to expand a little on some of the lens statistics and more importantly what they mean. This is by no means an exhaustive guide to lens buying, more just an explanation of the basics as I understand them.

Ok, I currently own 4 Sony lenses which Sony very handily list their statistics on their site so I will use these for comparisson. 

SEL-16F28, SEL-1855, SEL-30M35, SAL-55200-2

The lenses are:

SEL-16F28 - my new 'pancake' lens, a prime lens which is compact and good for travel etc.
SEL-1855 - my 'kit' lens, a zoom lens and by far my most frequently used lens so far.
SEL-30M35 - my macro lens, again a prime lens so no zoom.

SAL-55200-2 - my 'telephoto' lens, much greater zoom than the 1855 so used more for long distance shots.


The statistics of these lenses are:

Product Code	SEL-16F28	SEL-1855	SEL-30M35	SAL-55200-2
Product Name	E 16mm F2.8	E 18-55mm F3.5-5.6 OSS	E 30mm F3.5 Macro	DT 55-200mm F4-5.6 SAM
35mm equivalent focal lengthwhen used with APS-C sensor camera	24	27-82.5	45mm	82.5-300
Min. Focus (shortest distance from image sensor to subject); m	0.24	0.25	0.095	0.95
Dimension(Max. Diameter x Length); mm	62 x 22.5	62 x 60	62.0 x 55.5	71.5x85
Weight( approx.) ; g	67	194	138	305

The main statistics are:

Focal length - measured in mm. This is the measurement of the distance from the camera sensor to the lens glass and determines how wide an angle shot can be taken and also how much magnification the lens is capable of. While in older simpler camera lenses a 200mm lens would literally be 20cm long, modern optical systems use multiple lenses working in combination, which means that the light path can be shortened while still maintaining the same effective focal length.

A good description I read to explain this is to stand in a corridor with an open doorway at the end. The closer you are to that doorway, the more you can see of the room beyond. Compare my lenses on the table above and you can see that the pancake lens has the lowest focal length and therefore can see the most of the room beyond. 

Conversely, the telephoto lens has the highest focal length ranging from 55 to 200 depending on how much you zoom in with the lens. This is the equivalent of you standing right at the furthest point away from the doorway, you can only see a small amount of what is in that room.

Another analogy is that of pretending to be a pirate and looking through a tube. The eye end is that of the camera sensor, the other end is the end of the lens. The focal length represents the length of the tube - the longer the tube (higher focal length) the less you can see out of the other side.

The focal length also defines the magnification as well, a greater focal length = much greater magnification (similar to the pirate but using a real telescope).

That's great, but what does it mean for your pictures?

Traditionally with a 35mm film camera, the focal length of a 'standard' lens (one that pretty much takes a picture of what you see with the human eye) is approximately 50mm. If you had a 100mm lens, you would have approximately 2 x magnification, but only about half the angle of view.

Most digital cameras have a sensor that is smaller than a 35mm film frame, so these calculations can be performed by increase the focal length by x 1.5 so a 30mm digital lens is the equivalent of a 45mm on a film camera.

So for digital lenses, a 33mm lens is pretty much the human eye equivalent. 66mm is 2 x magnification but half the angle of view & so on.

Going back to my lenses, my 18-55mm kit lens ranges from nearly 1/2 de-magnification to 1.5 x magnification. Little wonder it is the main lens used as it has the most versatility around the 'standard' lens.

My 50-200 telephoto lens means it goes from a minimum of 1.5 x magnification to approximately 6x magnification. (& my Minolta 50-300mm lens extends to approx. 9x magnification)
Conversely my 16mm pancake lens is equivalent to 0.5 x magnification, so it fits quite a lot more into the picture than the human eye would.


F-stops - without going into a mass of detail about F-stops and how it is calculated, the F-stop figure represents the maximum aperture size the lens is capable of. This figure though is not a diameter, instead it is a ratio. The reason for this, if you imagine having a telephoto lens, it take less light to fill the lens when wide open than it does when at full zoom because the internal area of the lens is smaller. If this is hard to visualise, just image you were filling it with water, a fully zoomed lens has a greater capacity so requires more water. So saying that a lens had an aperture of 20mm diameter would still give different results depending on the internal dimensions of the lens.

Instead the figure is a ratio based on how many times the diameter of the aperture will fit into the length of the lens.

So, F2 means the diameter of the aperture will fit into the length of the lens twice.

F3 = 3 times and so on. 

What does this mean? Well, lets go back to filling the lens with water. If the lens is 50mm long, it only requires half as much water to fill it than if it was 100mm long.

The speed the water flows into the lens is restricted by the diameter of the aperture so the wider the aperture, the faster the water can enter the lens. If you had a 50mm lens with a 10mm diameter aperture, and you had a 100mm lens with a 20mm diameter aperture, then they would both fill at the same speed - so would both be the same F-stop. 

This is the key thing, the F-stop rating is a way of comparing different lenses capabilities. Regardless of the dimensions of the lens, an F2.8 lens is faster and therefore better at low light photography and usually gives sharper pictures, than an F3.5 lens. If you are looking for a specific type or size of lens you will see quite obviously that the lower the maximum F-stop rating, the more expensive they are! 

My new 16mm pancake lens is F2.8 compared to F3.5-5.6 for the 18-55mm kit lens, so the 16mm lens lets in more light which makes it a faster lens. What this means is that the aperture needs to stay open for a shorter time to correctly expose the picture, the less time the aperture is open for the sharper the resulting picture. 

Needless to say this also means the lens performs better in low light situations.

As I said, my 18-55mm kit lens is F3.5-5.6, this means that when the lens is 'wide open' at 18mm it has a maximum (lowest) F stop rating of 3.5, however when zoomed in to 55mm it only has an F-stop rating of 5.6, obviously when at 55mm the size of the area inside the lens is much greater so it takes longer to fill it with light.

The macro lens is F3.5 at 30mm, the same F-stop as my 18-55 at 18mm - so the macro lens is a much faster lens which is often greatly desirable for macro photography.

(for a fantastic full explanation of F stops, click here: http://www.uscoles.com/fstop.htm )

Minimum focus distance - this is quite self explanatory - measured in metres, this is the minimum distance at which the lens can focus (shortest distance from the image sensor to the subject).

My 18-55mm kit lens is 0.25, so the closest it can be to it's subject is 25cm
The 16mm pancake lens is 0.24 so pretty similar to the kit lens.
The 55-200mm telephoto lens is , so as you can see, you have to be further away from the subject in order to focus on it. 
The Macro lens is 0.095, so only 9.5cm distance between the image sensor and the subject.

I hope this is of use to some people, as said it is useful to aid in choosing lenses if you actually know what you're looking for.