Thursday, December 4, 2008

Lenses-How To Pick One And Read Their Specs: Part 1

Have you ever tried to purchase a lens but you had NO idea what to keep in mind or what to look for in it?

Maybe the store's salesperson tried to sell you the lens he/she wanted you to buy, showing you the numbers in front of the lens or the specs, but youre not sure that is the lens you need/want.

Have you ever researched for a lens and the full name of it was: Lensmaker 10-300mm f 2.8-8 Macro G APO DT SSM and you didnt understand what those things mean?

Well fear no more, this series of articles is going to give you a bearing of what all those things in a lens mean and what you should keep in mind when purchasing one.

The basics

There a few things you need to know before we begin:

  • The Sony Alpha DSLR system doesnt require stabilized lenses like other systems.

  • The Sony Alpha DSLR system uses a bayonet mount called Alpha mount which was inherited from Minolta.

  • The official lens maker for the Alpha DSLR system is Sony. Minolta AF lenses are also compatible and are considered official since the mount was originally developed by Minolta.

  • There are third party lens makers who make compatible Alpha mount lenses, such as Tamron, Sigma or Carl Zeiss, etc.

  • Lenses made for Minolta SLRs are full frame lenses (35mm film), they can be used with the Sony Alpha DSLRs as long as they are A(uto)F(ocus). Lenses made by Sony or other maker are full frame lenses unless specified different.

  • In the Alpha DSLR system, there are two sensor sizes: APS-C (23.6 x 15.7mm) and Full Frame (36 x 24mm), Full frame lenses can be used with Alpha DSLRs equipped with APS-C sized sensors, APS-C lenses can be used with full frame Alpha DSLRs but they cause vignetting or a darkening around the image. The vignetting occurs because APS-C lenses dont have the same field of view that Full Frame lenses do, resulting in cropping. Using these lenses on a full frame DSLR is possible but not recommended.

  • In the Alpha DSLR line the following cameras use APS-C sized sensors: A100/200/300/350/700. Cameras that use full frame sensors: A900.

Type of lenses

The first thing you need to determine when you look for a lens is what do you need it to do.

Do you need it for really close up shots? You want a lens that will let you include everyone when taking group pictures? You need a lens to get far into the scene without you moving?

There are 5 types of lenses:

  • Macro lenses, for close ups of really small things.

  • Wide angle lenses (and Super wide angle), they have a wider field of view so you can include more of the scene in the picture.

  • Zoom lenses, lenses with variable focal lengths that can let you take wide angle shots or really close pictures and anything in between.

  • Telephoto (and Super telephoto) lenses, these lenses are larger than Zoom lenses, they allow you to reach even farther without you physically moving.

  • Special effect lenses, These allow you to create inusual effects into your pictures that other lenses normally cant do, such as defocusing most of the image, altering the perspective, etc.

The macro, wide angle, telephoto and special effect lenses can have a zoom range in which you can move closer or away from your subject by moving a zoom ring in your lens but they can also be of fixed focal length, this means you cant move closer or away from your subject by moving a zoom ring in your lens, you got to physically move.

Reading a lens

Most lenses display their main specifications on the front, but not all of them, some don't due to their design or style.

The way the information is displayed also varies, Sony displays the information differently from Carl Zeiss on their lenses.

The information that lenses display on them is:

  • Maker

  • Focal length

  • Maximum constant aperture or variable depending on focal length

  • If it has a focusing engine built in

  • If its a higher grade lens (meaning better type of glass used and better image quality overall)

  • If its meant to be used specifically with APS-C sensors

  • Type of coatings used on the glass

  • If its a Macro, Reflex, Smooth Transition Focus lens etc.

Technical data not displayed in a lens

The lenses also have technical information that you should also keep in mind when purchasing it, this information is:

  • Lens groups/elements

  • Minimum focus distance

  • Filter diameter

  • Size

  • Weight

  • Minimum aperture

  • Angle of view

  • Number of aperture blades

  • If it has special glass elements or coatings

  • Magnification ratio

  • If it comes with any other accesories

Focal length multiplier

An important thing to keep in mind when purchasing a lens is the size of your sensor.

As mentioned above, the Alpha DSLR cameras use APS-C and Full frame sized sensors.

If you use a camera with an APS-C sensor, the effective focal length of your lens is modified because the APS-C sensor is smaller than the Full frame sensor.

This difference is called focal length multiplier.

What is this? In simple terms its how many times the length of the lens is multiplied taking in account the size of the sensor.

APS-C sensors are 1.5 times smaller than a full frame sensor, this means that the field of view of this sensor is reduced, and parts of the image that a full frame sensor can see, the APS-C sensor cant.

This translates that if you buy a 50mm lens, in an APS-C sensor the effective distance will be 75mm. 50 x 1.5 = 75

With telephoto lenses this is an advantage since you can get to the same place in less focal length than a full frame camera. If you use an APS-C sensor, you will get at 300 mm whereas a full frame camera needs 450 mm.

The downside is that wide angle lenses have to be designed especially for the APS-C sensor size, since full frame wide angle lenses dont yield the wider field of view on the smaller sensor. This translates in lenses with really short focal lengths (10-20mm for APS-C against 12-24mm for full frame) The problem is that they are expensive due to the complexity of their construction.

Sensor sizes:

Focal length multiplier:

Red for full frame, blue for APS-C

Images obtained from Wikipedia

In the following parts, the items listed above will be explained.

End of Part 1.


  1. Nice guide, and I found it useful. Thanks.

    Looking forward to the next part.

  2. Hello sillydog701!

    Im glad to know you found this guide useful, hopefully it will help you out when you need another lens in your arsenal :)

    Following parts will be coming soon.

    Thanks for visiting and commenting!