Choosing the Right ISO for Your Purpose

The prin­ci­pal func­tion of ISO and its influ­ence on effec­tive expo­sure is described in the intro­duc­tion to ISO and requires no repeat­ing. The pur­pose of this chap­ter is to acquaint you with the cor­rect method of using ISO to manip­u­late the effec­tive expo­sure of a pho­to­graph, the prac­ti­cal con­se­quences of rais­ing ISO val­ues, and, last­ly, some means for min­i­miz­ing the effects of image noise in soft­ware.

Correct ISO values

In dig­i­tal pho­tog­ra­phy, the best prac­tice for choos­ing the right ISO val­ue is to keep it as low as prac­ti­ca­ble in each sce­nario. In oth­er words, avoid set­ting an ISO val­ue that’s high­er than nec­es­sary to achieve the effect you desire from the aper­ture and/or shut­ter speed.

Aper­ture Pri­or­i­ty. If you’re using Aper­ture Pri­or­i­ty mode and the cam­era is indi­cat­ing a shut­ter speed that you know is two stops too slow for hand­held pho­tog­ra­phy, raise the ISO by two stops to achieve a faster shut­ter speed.

Shut­ter Pri­or­i­ty. If you’re using Shut­ter Pri­or­i­ty mode and the cam­era indi­cates that it can’t obtain a cor­rect expo­sure because it requires a larg­er aper­ture than your lens can achieve—indicated by the max­i­mum aper­ture val­ue blink­ing in the viewfinder—progressively raise the ISO until the f-num­ber stops flash­ing.

Reciprocity law and ISO

In the Reci­procity Law sec­tion, the rela­tion­ship between aper­ture and shut­ter speed was described as an inverse­ly pro­por­tion­al com­pro­mise between pic­tures that exhib­it a great depth of field and slow shut­ter speeds or shal­low depth of field and fast shut­ter speeds. The abil­i­ty to raise ISO val­ues in dig­i­tal pho­tog­ra­phy allows you to real­ize pho­tos show­ing both great depth of field (via small aper­tures) and frozen motion (via faster shut­ter speeds).

ISO priority mode

Except for Pen­tax, no cam­era man­u­fac­tur­er offers an explic­it­ly named ISO pri­or­i­ty mode. Nev­er­the­less, Pro­gram mode has exist­ed as the de fac­to ISO pri­or­i­ty mode on dig­i­tal cam­eras for many years. Pro­gram mode is an auto­mat­ic expo­sure mode in which the pho­tog­ra­ph­er selects a desired ISO val­ue, and the cam­era attempts to achieve ide­al effec­tive expo­sure by vary­ing both the aper­ture and shut­ter speed. Pro­gram mode is com­mon­ly indi­cat­ed as P on most cam­eras’ mode dials.

Auto ISO

In most dig­i­tal cam­eras, the ISO val­ue is auto­mat­i­cal­ly set in all ful­ly auto­mat­ic modes. In oth­er expo­sure modes, it’s pos­si­ble to des­ig­nate a range of ISO val­ues, from min­i­mum to max­i­mum, from which the cam­era auto­mat­i­cal­ly selects the most appro­pri­ate to achieve opti­mal effec­tive expo­sure.

Man­u­al mode. In Man­u­al mode, the aper­ture and shut­ter speed val­ues are set by the user. With ISO set to Auto, the cam­era will attempt to achieve opti­mal effec­tive expo­sure by auto­mat­i­cal­ly vary­ing the ISO with­in the pre­set Auto ISO range. Pen­tax is the first and only cam­era man­u­fac­tur­er to offer the func­tion­al­i­ty described above as a ded­i­cat­ed expo­sure mode, known as Shut­ter & Aper­ture Pri­or­i­ty (TAv).

Aper­ture Pri­or­i­ty. In Aper­ture Pri­or­i­ty mode, the aper­ture val­ue is set by the user, and the shut­ter speed val­ue is deter­mined by the cam­era. When con­fig­ur­ing Auto ISO, many cam­eras allow pho­tog­ra­phers to des­ig­nate a desired min­i­mum shut­ter speed. If the cam­era deter­mines that the ide­al effec­tive expo­sure requires a shut­ter speed val­ue that is slow­er than the set min­i­mum shut­ter speed, the cam­era will increase ISO with­in the Auto ISO range to reach the opti­mal effec­tive expo­sure.

Some cam­eras offer two pos­si­bil­i­ties for deter­min­ing the min­i­mum shut­ter speed, either deter­mined auto­mat­i­cal­ly by the cam­era or set man­u­al­ly by the user. When done auto­mat­i­cal­ly, the cam­era sets the min­i­mum shut­ter speed to the rec­i­p­ro­cal of the focal length of the lens, which fol­lows the min­i­mum shut­ter speed for­mu­la for hand­held pho­tog­ra­phy. (Some cam­eras offer fur­ther refine­ments to the method.) When set man­u­al­ly, the pho­tog­ra­ph­er selects a desired min­i­mum shut­ter speed, which the cam­era will main­tain regard­less of the focal length of the lens. In gen­er­al, the cam­era will only increase the shut­ter speed above the set min­i­mum when the scene becomes too bright to achieve ide­al effec­tive expo­sure using the min­i­mum ISO val­ue of the auto ISO range.

Shut­ter Pri­or­i­ty. In Shut­ter Pri­or­i­ty mode, the shut­ter speed is set by the user, and the aper­ture val­ue is deter­mined by the cam­era. When the cam­era deter­mines that the ide­al expo­sure requires an aper­ture that’s larg­er than your lens can achieve, the cam­era will increase ISO with­in the Auto ISO range to reach the opti­mal effec­tive expo­sure. Since the aper­ture range of a lens is much nar­row than the camera’s shut­ter speed range, the Auto ISO func­tion doesn’t per­mit set­ting an aper­ture lim­it.

Native ISO and expanded ISO

Most cam­eras offer two types of ISO val­ues, native and expand­ed. Native ISO con­sists of the ISO val­ues gen­er­at­ed through ampli­fi­ca­tion, or gain, of the image sensor’s ana­logue sig­nal. The major­i­ty of your camera’s full-stop ISO values—such as 100, 200, 400, and so forth—are native. In cam­eras that offer Expand­ed ISO, the Expand­ed ISO val­ues, often marked as high or low, are achieved by sim­u­lat­ing changes to expo­sure after the sig­nal has been dig­i­tized, typ­i­cal­ly via soft­ware adjust­ments to bright­ness. For exam­ple, when tak­ing a pho­to at ISO 12,800 on a cam­era whose high­est native ISO is 6,400, the cam­era will under­ex­pose an ISO 6,400 image, and sub­se­quent­ly lift its bright­ness by one stop in soft­ware; this process of inverse­ly pro­por­tion­al under­ex­pos­ing and bright­en­ing is known as push­ing. Con­verse­ly, when tak­ing a pho­to at ISO 50 on a cam­era whose low­est native ISO is 100, the cam­era will over­ex­pose an ISO 100 image, and then reduce its bright­ness by one stop in soft­ware; this process of inverse­ly pro­por­tion­al over­ex­pos­ing and dark­en­ing is known as pulling. Push­ing an image using Expand­ed ISO pro­duc­ers images with increased noise and less shad­ow detail. Pulling an image using Expand­ed ISO nom­i­nal­ly reduces image noise at the risk of los­ing details in the bright areas of the image to over­ex­po­sure.

Whether the inter­me­di­ate ISO val­ues are a prod­uct of ana­logue gain (native) or push­ing and pulling (expand­ed) is the cause of much online spec­u­la­tion. Every cam­era that per­mits user selec­tion of the inter­me­di­ate ISO val­ues includes them as part of the Native ISO range. If the spec­u­la­tors are cor­rect and the inter­me­di­ate ISO val­ues are derived by some vari­a­tion of push­ing or pulling of adja­cent Native ISOs, then the dif­fer­ence is only a max­i­mum of one-third stop, which should nei­ther result in a per­cep­ti­ble increase in noise or notice­able loss of high­light details.