Basic Camera Designs: DSLR and Mirrorless

This guide and our group pho­tog­ra­phy work­shops assume that your cam­era falls into one of two cat­e­gories, either an inter­change­able-lens cam­era (ILC) or an advanced enthu­si­ast-lev­el fixed-lens cam­era. 

Interchangeable-lens cameras

There are two types of dig­i­tal ILCs, dig­i­tal SLR and mir­ror­less. Let’s explore their basic fea­tures. 

Digital single-lens reflex cameras

At the time of writ­ing, dig­i­tal SLRs remain the top-sell­ing ILCs, a trend cor­rob­o­rat­ed by the cam­eras I most com­mon­ly encounter in my group pho­tog­ra­phy work­shops. The pri­ma­ry fea­ture of a dig­i­tal SLR is that its lens forms both the opti­cal image in the viewfind­er and the image pro­ject­ed onto the dig­i­tal sen­sor to make the pho­to­graph. In the film era, this fea­ture sep­a­rat­ed SLRs from more tra­di­tion­al viewfind­er cam­eras, which used sep­a­rate lens­es for their view­ing and pic­ture-tak­ing sys­tems. Employ­ing a sin­gle-lens design solved the prob­lem of parallax—where the appar­ent posi­tion and rota­tion of a sub­ject are dif­fer­ent when viewed from slight­ly off­set per­spec­tives. Since the sep­a­ra­tion dis­tance of the lens­es in two-lens viewfind­er cam­eras could be as much as the dis­tance between your two eyes, achiev­ing pre­cise fram­ing, espe­cial­ly of close sub­jects, was near­ly impos­si­ble. 

SLRs use a hinged mir­ror assem­bly to facil­i­tate using a sin­gle lens for both the view­ing and pic­ture-tak­ing sys­tems. The image in the opti­cal viewfind­er is formed when light pass­ing through the lens is reflect­ed up (hence reflex) towards a ground glass focus­ing screen by a mir­ror angled at 45° to the lens axis. A pen­taprism or pen­tamir­ror divert the image into the viewfinder’s eye­piece. When tak­ing a pho­to­graph, the mir­ror swings up and out of the light’s path let­ting it expose the dig­i­tal sen­sor. 

Advantages of digital SLR cameras:

  • Accu­rate com­po­si­tion. The viewfind­er dis­plays the image formed by the camera’s lens and faith­ful­ly rep­re­sents the focus, depth of field, and com­po­si­tion of the result­ing pho­to­graph.
  • No delay view­ing. The opti­cal nature of the sys­tem means that you see the image in real-time, with­out delay.
  • Opti­cal viewfind­er. The opti­cal viewfind­er ensures that you see the full tonal range of the scene with your eyes.
  • Fast aut­o­fo­cus. The “phase detec­tion” aut­o­fo­cus sys­tem in dig­i­tal SLRs focus­es on your sub­ject very quick­ly. 

Disadvantages of digital SLR cameras:

  • Big­ger and heav­ier. SLRs are gen­er­al­ly larg­er and heav­ier than equiv­a­lent cam­eras because the mir­ror requires a large vol­ume of space with­in the body of the cam­era.
  • Mechan­i­cal­ly com­plex. The mir­ror and its move­ment add sig­nif­i­cant mechan­i­cal com­plex­i­ty to the sys­tem, increas­ing its price.
  • Viewfind­er black­out. The image in the viewfind­er briefly dis­ap­pears because the mir­ror must lift out of the way to allow the light to expose the dig­i­tal sen­sor.
  • Mir­ror vibra­tion. The mirror’s quick move­ment can induce vibra­tion in the sys­tem at slow­er shut­ter speeds.
  • Larg­er, com­plex lens­es. The vol­ume of space occu­pied by the mir­ror and its com­po­nents mean there’s a long dis­tance between the back of the lens and the dig­i­tal sen­sor (flange focal dis­tance). In par­tic­u­lar, this makes the designs of wide-angle lens­es more com­plex. 

Mirrorless cameras

Mir­ror­less inter­change­able lens cam­eras (MILCs) are designed to improve upon some of the short­com­ings of dig­i­tal SLRs, and it’s accom­plished by replac­ing the opti­cal view­ing sys­tem (mir­ror, focus­ing screen, pen­taprism) with an elec­tron­ic viewfind­er (EVF), rear LCD, or both. Dur­ing reg­u­lar oper­a­tion, light pass­ing through the lens projects direct­ly onto the dig­i­tal image sen­sor, which gen­er­ates and trans­mits a live video stream to the view­ing sys­tem. When a pic­ture is tak­en, the dig­i­tal sen­sor briefly ceas­es trans­mis­sion of the video stream to record a pho­to­graph at full qual­i­ty. 

Advantages of mirrorless cameras:

  • Small­er and lighter. The cam­era bod­ies tend to be small­er and lighter than com­pa­ra­bly equipped dig­i­tal SLRs due to the absence of a mir­ror and its hous­ing.
  • Expo­sure pre­view. The view­ing sys­tem shows an accu­rate rep­re­sen­ta­tion of the image formed by the lens, and on many mod­els, can sim­u­late the final expo­sures, white bal­ance, and pic­ture pro­files (i.e., what you see is what you get).
  • Expo­sure aides. The view­ing sys­tems in advanced mod­els may present addi­tion­al aides such as expo­sure warn­ings, the his­togram, focus assist, and more.
  • Low-light view­ing. The view­ing sys­tem can pro­vide a brighter view than can oth­er­wise be achieved with an opti­cal viewfind­er (or the naked eye) in low-light con­di­tions.
  • Pre­cise aut­o­fo­cus. The con­trast detec­tion aut­o­fo­cus sys­tems used by many mir­ror­less cam­eras are typ­i­cal­ly more accu­rate in focus­ing the sub­ject. 

Disadvantages of mirrorless cameras:

  • Poor bat­tery per­for­mance. Because the elec­tron­ic view­ing sys­tem is con­tin­u­ous­ly engag­ing the dig­i­tal sen­sor, MILCs have much weak­er bat­tery per­for­mance than dig­i­tal SLRs.
  • EVF/LCD lag. Since the elec­tron­ic viewfind­er must process the video sig­nal from the dig­i­tal sen­sor and dis­play it for the user, there is a per­cep­ti­ble delay com­pared to the opti­cal viewfind­ers of SLRs. The degree of the delay varies between mod­els, with mod­ern high­er-end cam­eras hav­ing short­er lag times that bor­der on the imper­cep­ti­ble.
  • EVF/LCD res­o­lu­tion. Where­as the visu­al acu­ity of your eyes lim­its the res­o­lu­tion of the opti­cal sys­tems in dig­i­tal SLRs, small pix­el dimen­sions lim­it the resolv­ing pow­er of EVFs and LCDs of mir­ror­less cam­eras. This makes visu­al assess­ments of shal­low focus and depth of field dif­fi­cult and can pro­duce dis­tract­ing moiré pat­terns in fine­ly tex­tured sub­jects. 
  • Slow­er aut­o­fo­cus. Con­trast detec­tion aut­o­fo­cus, while accu­rate, is notice­ably slow­er than the “phase detec­tion” aut­o­fo­cus sys­tems used in all dig­i­tal SLRs and some high­er-end mir­ror­less cam­eras. 

Fixed-lens cameras

All fixed-lens cam­eras share two com­mon traits: 1) their lens­es are per­ma­nent­ly inte­grat­ed into the cam­era bod­ies and can’t be removed; 2) they are mir­ror­less by design. This mar­ket cat­e­go­ry encom­pass­es a tremen­dous vari­ety of styles, rang­ing from the very basic auto-every­thing point-and-shoots up to enthu­si­ast-lev­el com­pact cam­eras fea­tur­ing a full range of man­u­al expo­sure con­trols and more. This guide and our group pho­tog­ra­phy work­shops are intend­ed for pho­tog­ra­phers using the lat­ter type of fixed-lens cam­eras. A sim­ple method for deter­min­ing if your com­pact cam­era offers this lev­el of con­trol is to check whether there’s a mode dial on top of the cam­era with the fol­low­ing let­ters print­ed on it: M, S, A, and P (or M, Tv, Av, or P for Canon, Pen­tax, and Leica mod­els).1 There are a cou­ple of notable excep­tions to this rule, such as sev­er­al Fuji­film X series and Pana­son­ic LX100/Leica D‑Lux series com­pact cam­eras.

Advantages of fixed-lens cameras

  • They’re portable. Fixed-lens cam­eras tend to be very small and light com­pared to ILCs because they lack a mir­ror box, inte­grate lens­es phys­i­cal­ly deep­er into the cam­era body, and typ­i­cal­ly use small for­mat sen­sors. 
  • Ver­sa­tile lens­es. Most com­pact cam­eras with zoom lens­es fea­ture a ver­sa­tile range that can cap­ture both wide and nar­row angles of view. 
  • Grab and go. Every­thing you need as a pho­tog­ra­ph­er, whether it be a flash or a ver­sa­tile lens, is built-in. Lim­i­ta­tion, as it turns out, is free­ing; being lim­it­ed to the one fixed-lens means that you don’t have to delib­er­ate about what lens to bring for each excur­sion, which is a com­mon source of anx­i­ety for many pho­tog­ra­phers. 

Disadvantages of fixed-lens cameras

  • Poor lens options. You only get one oppor­tu­ni­ty to choose a lens, and it’s at the time of pur­chase. What if the lens you want comes on a cam­era you don’t? What if your pre­ferred cam­era doesn’t have the right lens? You’re out of luck if you desire a super wide-angle lens since no com­pact cam­eras have them. The oppo­site end of the zoom spec­trum is eas­i­er to accom­mo­date; there are sev­er­al com­pact cam­eras with very high mag­ni­fi­ca­tion zoom lens­es, although they tend to achieve such feats with sig­nif­i­cant com­pro­mis­es to image qual­i­ty.
  • Lim­it­ed selec­tive focus. A photographer’s abil­i­ty to selec­tive­ly focus on one sub­ject while simul­ta­ne­ous­ly allow­ing the rest of the scene fall out of focus is pri­mar­i­ly deter­mined by the com­bi­na­tion of their lens and cam­era. The qual­i­ty of how much of the scene is in focus is known as depth of field. In gen­er­al, the design trade-offs that allow fixed-lens cam­eras to be com­pact dimin­ish­es your abil­i­ty to iso­late your sub­ject from the back­ground with selec­tive focus.
  • Slow zoom­ing. The zoom mech­a­nisms in com­pact cam­eras tend to oper­ate on a fly-by-wire prin­ci­ple, where push­ing a but­ton or nudg­ing a lever engages an elec­tron­ic motor that dri­ves the zoom. These tend to work in dis­tinct steps, and not smooth­ly.  
  • Slow focus­ing. Assum­ing you have the option of man­u­al­ly focus­ing the lens, the focus mech­a­nisms in every com­pact cam­era are oper­at­ed by the fly-by-wire prin­ci­ples. Addi­tion­al­ly, most com­pact cam­eras use the con­trast detec­tion method for aut­o­fo­cus­ing their lens­es, which, although accu­rate, is rel­a­tive­ly slow.
  • Poor ergonom­ics for man­u­al con­trol. Increased automa­tion is a dou­ble-edged sword. Cam­eras designed for auto-every­thing oper­a­tion often sac­ri­fice your abil­i­ty to access man­u­al con­trols eas­i­ly.