Goals – Focus, Aberrations, Depth Of Field,Angle Of View
Buy a goal is not easy, especially when you have before a very high price, or, in an attempt to risparmaire, a used item.
Apart qualitatively assess the use (scratches, closing the diaphragm, dust and so on), when choosing a goal must put first emphasis on minimum focusing distance. This is because each lens has a different focal length the macro (which we will see below) are able to focus on just a few centimeters, the canvases to approximately 1.5 meters, the catadioptric around 4-5 meters. In short, you have to have in mind what you want to do If your goal is to photograph landscapes, the focus distance will be imported shortly.
A second parameter to consider is the location of the focus ring to focus. Do not assume that you will not need the autofocus does not always work and there are situations, especially in low light, where it goes into crisis http://www.aerozoneasia.com/our-services/aerial-surveymapping. Have easy access to the ring (but also to switch on off AF) it is critical. In some goals the ring is positioned near term target, in other close to the body what is better It depends on how you are. Attention to the hood I used to mount it on the contrary as I do not use it and, in some cases, could go to cover the ring in question. Very uncomfortable especially if you have no place to throw the hood. Also on the bezel, check with the movement carefully it must be fluid, without resistance and not noisy. Although the accuracy is a parameter to be considered a too soft ring is counterproductive, as well as a too tough nut.
Regarding the position of the focus ring (close to the camera body or at the end of the lens), it should be stated that this choice is directly linked to the element which is moved to achieve focusing. When the nut is positioned to the extreme (in most modern zoom), to move a lens is rather thin and light the precision is traditionally greater, than the speed of the operation. In other objectives, to move is a big lens positioned close to the machine.While in the first case the excursion is very small, in the second case is much greater which creates two problems the first is that the lens acts as a plunger if there is a loss in the lens body, this movement it will bring all ‘indoor air and dust.